diff --git a/applications/test/OCharStream1/Test-OCharStream1.cxx b/applications/test/OCharStream1/Test-OCharStream1.cxx
index b5def1b8fa..2474cfd5df 100644
--- a/applications/test/OCharStream1/Test-OCharStream1.cxx
+++ b/applications/test/OCharStream1/Test-OCharStream1.cxx
@@ -183,12 +183,25 @@ int main(int argc, char *argv[])
     printInfo(obuf);
 
     // Overwrite at some position
-    obuf.stdStream().rdbuf()->pubseekpos(0.60 * obuf.size());
-    obuf << "<" << nl << "OVERWRITE" << nl;
+    if (auto i = obuf.view().find("item5"); i != std::string::npos)
+    {
+        // obuf.seek(0.60 * obuf.size());
+        obuf.seek(i);
+        obuf << "<OVERWRITE>" << nl;
+    }
 
     Info<<"after overwrite" << nl;
     printInfo(obuf);
 
+    // Truncate
+    {
+        constexpr float fraction = 0.90;
+        Info<<"truncated at " << (100*fraction) << "% ["
+            << int(fraction*obuf.size()) << " chars]" << nl;
+        obuf.seek(fraction*obuf.size());
+        printInfo(obuf);
+    }
+
     Info<< "transfer contents to a List or ICharStream" << nl;
 
     // Reclaim data storage from OCharStream -> ICharStream
diff --git a/applications/test/OCharStream2/Make/files b/applications/test/OCharStream2/Make/files
new file mode 100644
index 0000000000..e2ffcd6203
--- /dev/null
+++ b/applications/test/OCharStream2/Make/files
@@ -0,0 +1,3 @@
+Test-OCharStream2.cxx
+
+EXE = $(FOAM_USER_APPBIN)/Test-OCharStream2
diff --git a/applications/test/OCharStream2/Make/options b/applications/test/OCharStream2/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/OCharStream2/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/OCharStream2/Test-OCharStream2.cxx b/applications/test/OCharStream2/Test-OCharStream2.cxx
new file mode 100644
index 0000000000..5c66990c41
--- /dev/null
+++ b/applications/test/OCharStream2/Test-OCharStream2.cxx
@@ -0,0 +1,473 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Description
+
+\*---------------------------------------------------------------------------*/
+
+#include "SpanStream.H"
+#include "wordList.H"
+#include "IOstreams.H"
+#include "argList.H"
+
+#include <charconv>
+#include <cctype>
+#include <cstdio>
+#include <limits>
+#include <iomanip>
+
+using namespace Foam;
+
+Ostream& printString(Ostream& os, const char* first, const char* last)
+{
+    os << '"';
+    for (; first != last; (void)++first)
+    {
+        os << *first;
+    }
+    os << '"';
+
+    return os;
+}
+
+
+Ostream& printView(Ostream& os, const char* first, const char* last)
+{
+    char buf[4];
+    os << label(last-first) << '(';
+
+    for (; first != last; (void)++first)
+    {
+        const char c = *first;
+
+        if (isprint(c))
+        {
+            os << c;
+        }
+        else if (c == '\t')
+        {
+            os << "\\t";
+        }
+        else if (c == '\n')
+        {
+            os << "\\n";
+        }
+        else
+        {
+            ::snprintf(buf, 4, "%02X", c);
+            os << "\\x" << buf;
+        }
+    }
+    os << ')';
+
+    return os;
+}
+
+
+Ostream& printView(Ostream& os, std::string_view s)
+{
+    return printView(os, s.begin(), s.end());
+}
+
+
+Ostream& printView(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list.begin(), list.end());
+}
+
+
+Ostream& writeList(Ostream& os, const UList<char>& list)
+{
+    return printView(os, list);
+}
+
+
+Ostream& toString(Ostream& os, const UList<char>& list)
+{
+    return printString(os, list.begin(), list.end());
+}
+
+
+Ostream& toString(Ostream& os, std::string_view s)
+{
+    return printString(os, s.begin(), s.end());
+}
+
+
+template<class BufType>
+void printInfo(const BufType& buf)
+{
+    Info<< nl << "=========================" << endl;
+    buf.print(Info);
+    Info<< "addr: " << Foam::name(buf.view().data()) << nl;
+    toString(Info, buf.view());
+    Info<< nl << "=========================" << endl;
+}
+
+
+// Return a left-padded integer as "word"
+template<class IntType>
+std::string leftpadded(IntType val, char fillch = ' ')
+{
+    std::string buf;
+    buf.resize((std::numeric_limits<IntType>::digits10+1), fillch);
+
+    auto first = (buf.data());
+    auto last  = (buf.data() + buf.size());
+
+    auto result = std::to_chars(first, last, val);
+
+    if (result.ec == std::errc{})
+    {
+        auto* iter = result.ptr;
+        int count = std::distance(iter, last);
+
+        std::cout << "did not fill: " << count << " chars\n";
+
+        // With two spaces before comments
+        if (count > 0) { *iter++ = ' '; --count; }
+        if (count > 0) { *iter++ = ' '; --count; }
+        for (char c = (count >= 2 ? '/' : ' '); count > 0; --count)
+        {
+            *iter++ = c;
+        }
+    }
+
+    return buf;
+}
+
+
+template<class IntType>
+void leftpad(std::ostream& os, IntType val, char fillch = ' ')
+{
+    // set fill char and width
+    os.setf(std::ios_base::left, std::ios_base::adjustfield);
+    fillch = os.fill(fillch);
+    os.width(std::numeric_limits<IntType>::digits10+1);
+    os  << val;
+    // restore fill char
+    os.fill(fillch);
+}
+
+
+template<class IntType>
+void rightpad(std::ostream& os, IntType val, char fillch = ' ')
+{
+    // set fill char and width
+    os.setf(std::ios_base::right, std::ios_base::adjustfield);
+    fillch = os.fill(fillch);
+    os.width(std::numeric_limits<IntType>::digits10+1);
+    os  << val;
+    // restore fill char
+    os.fill(fillch);
+}
+
+
+// Left-padded value with trailing comment slashes
+template<class IntType>
+void leftpad(ocharstream& os, IntType val)
+{
+    const auto beg = os.tellp();
+    os << val;
+    int count = (std::numeric_limits<IntType>::digits10+1) - (os.tellp() - beg);
+
+    // With two spaces before comments
+    if (count > 0) { os << ' '; --count; }
+    if (count > 0) { os << ' '; --count; }
+    for (const char c = (count >= 2 ? '/' : ' '); count > 0; --count)
+    {
+        os << c;
+    }
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    argList::noBanner();
+    argList::noParallel();
+    argList::addBoolOption("fake-zerosize", "Fake overwriting with zero data");
+    argList::addBoolOption("dict-format", "Format as dictionary entry");
+
+    #include "setRootCase.H"
+
+    const bool optFakeZerosize = args.found("fake-zerosize");
+    const bool isDictFormat = args.found("dict-format");
+
+    // const constexpr int width = (std::numeric_limits<label>::digits10+1);
+
+    // experiment with to_chars instead of streaming
+    {
+        // Some value
+        label val(1234);
+
+        auto fixed = leftpadded(val);
+        Info<< "leftpadded " << val << " : " << fixed << nl;
+    }
+
+    ocharstream labelbuf;
+    labelbuf.reserve_exact(32);
+
+    // Some value
+    labelbuf.rewind();
+    rightpad(labelbuf, label(10));
+
+    printInfo(labelbuf);
+
+    OCharStream obuf;
+    obuf.reserve_exact(48);
+
+    printInfo(obuf);
+
+    obuf.push_back('>');
+    obuf.append(" string_view ");
+    obuf.push_back('<');
+    printInfo(obuf);
+
+    obuf.pop_back(8);
+    printInfo(obuf);
+
+    obuf.pop_back(100);
+    printInfo(obuf);
+
+
+    // Fill with some content
+    for (int i = 0; i < 26; ++i)
+    {
+        obuf<< char('A' + i);
+    }
+
+    // Change letter 'O' to '_'
+    if (auto i = obuf.view().find('O'); i != std::string::npos)
+    {
+        obuf.overwrite(i, '_');
+    }
+
+    // append and push_back some content
+    obuf.append(5, '<');
+    obuf.push_back('#');
+    obuf.append(5, '>');
+
+    printInfo(obuf);
+
+    obuf.pop_back(8);
+    printInfo(obuf);
+
+    // Slightly silly test
+    {
+        const auto list = obuf.list();
+        Info<< "list content:" << list << nl;
+        Info<< "view content:" << nl << list.view() << nl;
+    }
+
+    obuf.overwrite(4, labelbuf.view());
+    printInfo(obuf);
+
+    obuf.overwrite(20, "####");
+    printInfo(obuf);
+
+    Info<< "operation Ignored..." << nl;
+    obuf.overwrite(45, "????");
+    printInfo(obuf);
+
+    // Update with new value
+    {
+        labelbuf.rewind();
+        rightpad(labelbuf, label(200), '.');
+        obuf.overwrite(4, labelbuf.view());
+
+        printInfo(obuf);
+    }
+
+    // With yet another value (non-fixed width)
+    {
+        labelbuf.rewind();
+        labelbuf << label(15);
+        obuf.overwrite(4, labelbuf.view());
+
+        printInfo(obuf);
+    }
+
+    // Slightly harder test
+    {
+        std::string chars(26, '?');
+
+        for (int i = 0; i < 26; ++i)
+        {
+            chars[i] = char('A' + i);
+        }
+
+        auto& os = obuf;
+        os.rewind();
+
+        const word procName("processor0");
+
+        // Write as primitiveEntry or commented content
+        // // constexpr bool isDictFormat = true;
+
+        // if constexpr (isDictFormat)
+        if (isDictFormat)
+        {
+            // Like writeKeyword() with compoundToken
+            os << nl << procName << ' ' << word("List<char>") << nl;
+        }
+        else
+        {
+            // Human-readable comments
+            os << nl << "// " << procName << nl;
+        }
+
+        // This is the code we want to have, but assume we don't know
+        // the size or data beforehand.
+        //
+        // if (str && len > 0)
+        // {
+        //     // Special treatment for char data (binary I/O only)
+        //     const auto oldFmt = os.format(IOstreamOption::BINARY);
+        //
+        //     os << label(len) << nl;
+        //     os.write(str, len);
+        //     os << nl;
+        //
+        //     os.format(oldFmt);
+        // }
+        // else
+        // {
+        //     os << label(0) << nl;
+        // }
+
+        // Position before writing the label
+        const auto labelBegin = os.tellp();
+
+        // Replace: os << label(len) << nl;
+        // with a fixed-length version
+        {
+            labelbuf.rewind();
+            rightpad(labelbuf, 0);
+
+            os.append(labelbuf.view());
+            os << nl;
+        }
+
+        constexpr bool testUnknown = true;
+
+        label dataCount = 0;
+
+        if constexpr (testUnknown)
+        {
+            // Pretend we don't know the number of characters a priori
+
+            const auto oldFmt = os.format(IOstreamOption::BINARY);
+
+            const auto lineNumber = os.lineNumber();
+
+            // count is unknown but irrelevant for serial
+            os.beginRawWrite(0);
+
+            // Position before raw binary data
+            const auto dataBegin = os.tellp();
+
+            // Some type of output, streaming etc
+            os.writeRaw(chars.data(), chars.size());
+
+            // How many chars of binary data written?
+            dataCount = (os.tellp() - dataBegin);
+
+            os.endRawWrite();
+            os.lineNumber() = lineNumber;
+            os << nl;
+
+            os.format(oldFmt);
+        }
+        else
+        {
+            // If we had all data collected a priori
+
+            dataCount = chars.size();
+
+            const auto oldFmt = os.format(IOstreamOption::BINARY);
+
+            if (dataCount > 0)
+            {
+                os.write(chars.data(), chars.size());
+                os << nl;
+            }
+            os.format(oldFmt);
+        }
+
+        if (optFakeZerosize)
+        {
+            dataCount = 0;  // fake zero-size
+        }
+
+        printInfo(os);
+
+        // Update the data count with the correct value
+
+        if (dataCount > 0)
+        {
+            labelbuf.rewind();
+            leftpad(labelbuf, label(dataCount));
+
+            os.overwrite(labelBegin, labelbuf.view());
+        }
+        else
+        {
+            os.seek(int64_t(labelBegin)-1);
+
+            // if constexpr (isDictFormat)
+            if (isDictFormat)
+            {
+                os << ' ' << label(0);
+            }
+            else
+            {
+                os << nl << label(0) << nl;
+            }
+        }
+
+        // if constexpr (isDictFormat)
+        if (isDictFormat)
+        {
+            os.endEntry();
+        }
+
+        printInfo(os);
+
+        Info<< "view: " << os.view(4, 8) << nl;
+        Info<< "view: " << os.view(32) << nl;
+        // Ignores out-of-range
+        Info<< "view: " << os.view(1000) << nl;
+    }
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/applications/test/parallel-file-write1/Make/files b/applications/test/parallel-file-write1/Make/files
new file mode 100644
index 0000000000..7026affab8
--- /dev/null
+++ b/applications/test/parallel-file-write1/Make/files
@@ -0,0 +1,3 @@
+Test-parallel-file-write1.cxx
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-file-write1
diff --git a/applications/test/parallel-file-write1/Make/options b/applications/test/parallel-file-write1/Make/options
new file mode 100644
index 0000000000..18e6fe47af
--- /dev/null
+++ b/applications/test/parallel-file-write1/Make/options
@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */
diff --git a/applications/test/parallel-file-write1/Test-parallel-file-write1.cxx b/applications/test/parallel-file-write1/Test-parallel-file-write1.cxx
new file mode 100644
index 0000000000..5d92636ae5
--- /dev/null
+++ b/applications/test/parallel-file-write1/Test-parallel-file-write1.cxx
@@ -0,0 +1,260 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-parallel-file-write1
+
+Description
+    Simple test of writing with MPI/IO
+
+\*---------------------------------------------------------------------------*/
+
+#include "argList.H"
+#include "Time.H"
+#include "Switch.H"
+#include "UPstreamFile.H"
+#include "SpanStream.H"
+
+using namespace Foam;
+
+template<class IntType>
+void zeropadded(std::ostream& os, IntType val, char fillch = '0')
+{
+    // set fill char and width
+    os.setf(std::ios_base::right, std::ios_base::adjustfield);
+    fillch = os.fill(fillch);
+    os.width(std::numeric_limits<IntType>::digits10+1);
+    os  << val;
+    // restore fill char
+    os.fill(fillch);
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char *argv[])
+{
+    argList::noCheckProcessorDirectories();
+    argList::addVerboseOption();
+    argList::addBoolOption("master-footer", "Write footer from master");
+
+    #include "setRootCase.H"
+
+    if (!UPstream::parRun())
+    {
+        Info<< "###############" << nl
+            << "Not running in parallel. Stopping now" << nl
+            << "###############" << endl;
+        return 1;
+    }
+
+    const bool optMasterFooter = args.found("master-footer");
+
+    Info<< nl << "Write master-footer: " << Switch::name(optMasterFooter)
+        << nl << nl;
+
+    Info<< "Create time (without controlDict)\n" << endl;
+
+    auto runTimePtr = Time::New();
+    auto& runTime = runTimePtr();
+
+    const auto myProc = UPstream::myProcNo();
+    const auto nProcs = UPstream::nProcs();
+
+    // Some content
+    OCharStream charset;
+    for (int i = 0; i < 10; ++i)
+    {
+        charset<< char('A' + i);
+    }
+
+    // Header/footer buffers - these can be separate or bundled into
+    // the first/last blocks
+
+    OCharStream header;
+    OCharStream footer;
+
+    // Content buffer
+    OCharStream os(IOstream::BINARY);
+
+    {
+        const auto v = charset.view();
+
+        os << nl;
+        os.beginBlock(word("rank" + Foam::name(myProc)));
+
+        for (int repeat = 0;  repeat <= myProc; ++repeat)
+        {
+            os << indent << word("entry" + Foam::name(repeat))
+                << ' ' << word("List<char>");
+            // os << nl;
+            os << ' ';
+            os << label(v.size());
+            os.write(v.data(), v.size());
+            // os << nl;
+            os.endEntry();
+        }
+
+        os.endBlock();
+    }
+
+    // Bundle the footer into the last block
+    if (!optMasterFooter && (myProc == nProcs-1))
+    {
+        IOobject::writeEndDivider(os);
+    }
+
+
+    // All content now exists - commit to disk
+    const std::string_view blockData(os.view());
+    const int64_t blockSize(blockData.size());
+
+    // Collect sizes
+    const List<int64_t> sizes
+    (
+        UPstream::allGatherValues(blockSize, UPstream::worldComm)
+    );
+
+
+    // Format header with size information
+    if (UPstream::master())
+    {
+        header
+            << "Simple MPI/IO test with " << nProcs << " ranks" << nl << nl;
+
+        ocharstream labelbuf;
+        labelbuf.reserve_exact(32);
+
+        // Position before writing a label
+        auto labelBegin = header.tellp();
+
+        header.beginBlock("meta");
+        {
+            header << indent << word("data.start") << ' ';
+
+            labelBegin = header.tellp();
+
+            // Add the start value (placeholder)
+            {
+                labelbuf.rewind();
+                zeropadded(labelbuf, label(0));
+                header.append(labelbuf.view());
+            }
+
+            header.endEntry();
+
+            header << indent << word("data.sizes") << nl;
+            sizes.writeList(header);  // flatOutput
+            header.endEntry();
+        }
+        header.endBlock();
+
+        header << nl;
+        IOobject::writeDivider(header);
+
+        // Now update with the correct size
+        {
+            labelbuf.rewind();
+            zeropadded(labelbuf, label(header.view().size()));
+            header.overwrite(labelBegin, labelbuf.view());
+        }
+
+        // Bundled the footer into the last block or from master?
+        if (optMasterFooter)
+        {
+            IOobject::writeEndDivider(footer);
+        }
+    }
+
+    // With additional header/footer
+    int64_t headerSize(header.view().size());
+    int64_t footerSize(footer.view().size());
+
+    Pstream::broadcast(headerSize);
+    if (optMasterFooter)
+    {
+        Pstream::broadcast(footerSize);
+    }
+
+
+    int64_t totalSize(headerSize);
+    for (int i = 0; i < myProc; ++i)
+    {
+        totalSize += sizes[i];
+    }
+
+    const int64_t blockOffset(totalSize);
+
+    for (int i = myProc; i < nProcs; ++i)
+    {
+        totalSize += sizes[i];
+    }
+    const int64_t footerOffset(totalSize);
+    totalSize += footerSize;
+
+
+    Pout<< "write size=" << label(blockSize)
+        << " at=" << label(blockOffset) << " total=" << label(totalSize) << nl;
+
+    {
+        UPstream::File file;
+
+        bool ok = file.open_write
+        (
+            UPstream::worldComm,
+            runTime.globalPath()/"mpiio-test1.txt",
+            IOstreamOption::ATOMIC
+        );
+
+        if (ok)
+        {
+            Info<< "writing: " << file.name() << nl;
+
+            if (UPstream::master())
+            {
+                // A no-op for empty buffer
+                ok = file.write_at(0, header.view());
+            }
+
+            ok = file.write_at_all(blockOffset, blockData);
+
+            if (UPstream::master())
+            {
+                // A no-op for empty buffer
+                ok = file.write_at(footerOffset, footer.view());
+            }
+        }
+
+        file.set_size(totalSize);
+        file.close();
+    }
+
+    Info<< "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/containers/Lists/DynamicList/DynamicList.H b/src/OpenFOAM/containers/Lists/DynamicList/DynamicList.H
index 92932c92ba..2e92c83e3c 100644
--- a/src/OpenFOAM/containers/Lists/DynamicList/DynamicList.H
+++ b/src/OpenFOAM/containers/Lists/DynamicList/DynamicList.H
@@ -91,19 +91,14 @@ class DynamicList
         template<class ListType>
         inline void doAssignDynList(const ListType& list);
 
-        //- Alter the size of the underlying storage
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doCapacity(const bool nocopy, const label len);
+        //- Alter the size of the underlying storage,
+        //- retaining the first count elements.
+        inline void doCapacity_copy(label count, const label len);
 
-        //- Reserve allocation space for at least this size.
+        //- Reserve allocation space for at least this size,
+        //- retaining the first count elements.
         //  Never shrinks the allocated size, use setCapacity() for that.
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doReserve(const bool nocopy, const label len);
-
-        //- Reserve allocation space for at least this size.
-        //  Never shrinks the allocated size, use setCapacity() for that.
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doResize(const bool nocopy, const label len);
+        inline void doReserve_copy(label count, const label len);
 
         //- Read List from Istream between '(' and ')' delimiters.
         //- The size is not known a priori.
@@ -232,6 +227,10 @@ public:
         //- Alter addressable size and fill \em new entries with constant value
         inline void resize(const label len, const T& val);
 
+        //- Alter addressable size, retaining the first count contents.
+        //  \note Only uses a limited number of internal checks.
+        void resize_copy(label count, const label len);
+
         //- Alter addressable size and set val for \em all addressed entries
         inline void resize_fill(const label len, const T& val);
 
diff --git a/src/OpenFOAM/containers/Lists/DynamicList/DynamicListI.H b/src/OpenFOAM/containers/Lists/DynamicList/DynamicListI.H
index ed94ae38b4..bd2b6ffe8c 100644
--- a/src/OpenFOAM/containers/Lists/DynamicList/DynamicListI.H
+++ b/src/OpenFOAM/containers/Lists/DynamicList/DynamicListI.H
@@ -54,9 +54,9 @@ inline void Foam::DynamicList<T, SizeMin>::doAssignDynList
 
 
 template<class T, int SizeMin>
-inline void Foam::DynamicList<T, SizeMin>::doCapacity
+inline void Foam::DynamicList<T, SizeMin>::doCapacity_copy
 (
-    const bool nocopy,
+    label count,
     const label newCapacity
 )
 {
@@ -68,16 +68,22 @@ inline void Foam::DynamicList<T, SizeMin>::doCapacity
     // Addressable length, possibly truncated by new capacity
     const label currLen = Foam::min(List<T>::size(), newCapacity);
 
+    // The count truncated by the new addressable range
+    if (count > currLen)
+    {
+        count = currLen;
+    }
+
     // Consistent allocated sizing
     List<T>::setAddressableSize(capacity_);
 
-    if (nocopy)
+    if (count > 0)
     {
-        List<T>::resize_nocopy(newCapacity);
+        List<T>::resize_copy(count, newCapacity);
     }
     else
     {
-        List<T>::resize_copy(currLen, newCapacity);
+        List<T>::resize_nocopy(newCapacity);
     }
 
     capacity_ = List<T>::size();
@@ -86,9 +92,9 @@ inline void Foam::DynamicList<T, SizeMin>::doCapacity
 
 
 template<class T, int SizeMin>
-inline void Foam::DynamicList<T, SizeMin>::doReserve
+inline void Foam::DynamicList<T, SizeMin>::doReserve_copy
 (
-    const bool nocopy,
+    label count,
     const label len
 )
 {
@@ -97,20 +103,27 @@ inline void Foam::DynamicList<T, SizeMin>::doReserve
         // Preserve addressed size
         const label currLen = List<T>::size();
 
+        // The count truncated by the addressable range
+        if (count > currLen)
+        {
+            count = currLen;
+        }
+
         // Consistent allocated sizing
         List<T>::setAddressableSize(capacity_);
 
         // Increase capacity (eg, doubling)
+        // - this may need better handling for when lists become very large
         capacity_ =
             Foam::ListPolicy::reserve_size<SizeMin, 2>(len, capacity_);
 
-        if (nocopy)
+        if (count > 0)
         {
-            List<T>::resize_nocopy(capacity_);
+            List<T>::resize_copy(count, capacity_);
         }
         else
         {
-            List<T>::resize_copy(currLen, capacity_);
+            List<T>::resize_nocopy(capacity_);
         }
 
         capacity_ = List<T>::size();
@@ -119,18 +132,6 @@ inline void Foam::DynamicList<T, SizeMin>::doReserve
 }
 
 
-template<class T, int SizeMin>
-inline void Foam::DynamicList<T, SizeMin>::doResize
-(
-    const bool nocopy,
-    const label len
-)
-{
-    this->doReserve(nocopy, len);
-    List<T>::setAddressableSize(len);
-}
-
-
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 template<class T, int SizeMin>
@@ -323,7 +324,8 @@ inline void Foam::DynamicList<T, SizeMin>::setCapacity
     const label len
 )
 {
-    this->doCapacity(false, len);  // nocopy = false
+    // Preserve all content
+    doCapacity_copy(List<T>::size(), len);
 }
 
 
@@ -333,7 +335,8 @@ inline void Foam::DynamicList<T, SizeMin>::setCapacity_nocopy
     const label len
 )
 {
-    this->doCapacity(true, len);  // nocopy = true
+    // Preserve 0 content
+    doCapacity_copy(0, len);
 }
 
 
@@ -343,7 +346,8 @@ inline void Foam::DynamicList<T, SizeMin>::reserve
     const label len
 )
 {
-    this->doReserve(false, len);  // nocopy = false
+    // Preserve all content
+    doReserve_copy(List<T>::size(), len);
 }
 
 
@@ -353,7 +357,8 @@ inline void Foam::DynamicList<T, SizeMin>::reserve_nocopy
     const label len
 )
 {
-    this->doReserve(true, len);  // nocopy = true
+    // Preserve 0 content
+    doReserve_copy(0, len);
 }
 
 
@@ -371,9 +376,12 @@ inline void Foam::DynamicList<T, SizeMin>::reserve_exact
         // Consistent allocated sizing
         List<T>::setAddressableSize(capacity_);
 
+        // Exact length
+        capacity_ = len;
+
         // if (!nocopy)
         {
-            List<T>::resize_copy(currLen, len);
+            List<T>::resize_copy(currLen, capacity_);
         }
 
         capacity_ = List<T>::size();
@@ -388,7 +396,21 @@ inline void Foam::DynamicList<T, SizeMin>::resize
     const label len
 )
 {
-    this->doResize(false, len);  // nocopy = false
+    // Preserve all content
+    resize_copy(List<T>::size(), len);
+}
+
+
+template<class T, int SizeMin>
+inline void Foam::DynamicList<T, SizeMin>::resize_copy
+(
+    label count,
+    const label len
+)
+{
+    // Preserve count elements
+    doReserve_copy(count, len);
+    List<T>::setAddressableSize(len);
 }
 
 
@@ -399,7 +421,7 @@ inline void Foam::DynamicList<T, SizeMin>::resize_fill
     const T& val
 )
 {
-    this->doResize(true, len);  // nocopy = true
+    resize_nocopy(len);
     UList<T>::operator=(val);
 }
 
@@ -410,7 +432,8 @@ inline void Foam::DynamicList<T, SizeMin>::resize_nocopy
     const label len
 )
 {
-    this->doResize(true, len);  // nocopy = true
+    // Preserve 0 content
+    resize_copy(0, len);
 }
 
 
@@ -422,6 +445,8 @@ inline void Foam::DynamicList<T, SizeMin>::resize
 )
 {
     const label oldLen = List<T>::size();
+
+    // Preserve all content
     resize(len);
 
     // Fill newly exposed with constant value
@@ -445,6 +470,9 @@ inline void Foam::DynamicList<T, SizeMin>::clear() noexcept
 template<class T, int SizeMin>
 inline void Foam::DynamicList<T, SizeMin>::clearStorage()
 {
+    // Extra safety...?
+    if (!List<T>::cdata() && List<T>::empty()) capacity_ = 0;
+
     // Consistent allocated sizing
     List<T>::setAddressableSize(capacity_);
     List<T>::clear();
@@ -515,7 +543,7 @@ inline void Foam::DynamicList<T, SizeMin>::swap
     UList<T>::swap(other);
 
     // Swap capacity
-    std::swap(this->capacity_, other.capacity_);
+    std::swap(capacity_, other.capacity_);
 }
 
 
@@ -537,7 +565,7 @@ template<int AnySizeMin>
 inline void
 Foam::DynamicList<T, SizeMin>::transfer
 (
-    DynamicList<T, AnySizeMin>& list
+    DynamicList<T, AnySizeMin>& other
 )
 {
     if
@@ -545,7 +573,7 @@ Foam::DynamicList<T, SizeMin>::transfer
         FOAM_UNLIKELY
         (
             static_cast<const List<T>*>(this)
-         == static_cast<const List<T>*>(&list)
+         == static_cast<const List<T>*>(&other)
         )
     )
     {
@@ -554,10 +582,11 @@ Foam::DynamicList<T, SizeMin>::transfer
 
     // Consistent allocated sizing
     List<T>::setAddressableSize(capacity_);
-    List<T>::transfer(static_cast<List<T>&>(list));
+    List<T>::transfer(static_cast<List<T>&>(other));
 
-    capacity_ = list.capacity();
-    list.setCapacity_unsafe(0);  // All contents moved
+    // Update capacity information
+    capacity_ = other.capacity();
+    other.setCapacity_unsafe(0);  // All contents moved
 }
 
 
diff --git a/src/OpenFOAM/containers/Lists/List/List.C b/src/OpenFOAM/containers/Lists/List/List.C
index fa1e680e9c..bec1e08157 100644
--- a/src/OpenFOAM/containers/Lists/List/List.C
+++ b/src/OpenFOAM/containers/Lists/List/List.C
@@ -28,27 +28,38 @@ License
 
 #include "List.H"
 #include "FixedList.H"
-#include "PtrList.H"
+#include "UPtrList.H"
 
-// * * * * * * * * * * * * Protected Member Functions  * * * * * * * * * * * //
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
 
+// Only a limited number of internal size checks.
+// Caller knows what they are doing.
 template<class T>
-void Foam::List<T>::resize_copy(const label count, const label len)
+void Foam::List<T>::resize_copy(label count, const label len)
 {
-    // Only a limited number of internal size checks.
-    // Caller knows what they are doing.
-
-    if (FOAM_LIKELY(len > 0))
+    if (this->size_ == len)
+    {
+        // no-op
+    }
+    else if (FOAM_LIKELY(len > 0))
     {
         // With sign-check to avoid spurious -Walloc-size-larger-than
-        const label oldLen = this->size_;
-        const label overlap = Foam::min(count, len);
-        // Extra safety, not currently necessary:
-        // const label overlap = Foam::min(Foam::min(count, oldLen), len);
 
         T* old = this->v_;
+        const label oldLen = this->size_;
 
-        if (overlap > 0)
+        if (count > len)
+        {
+            count = len;        // The count truncated by the new length
+        }
+
+        // Extra safety, probably not necessary:
+        // if (count > oldLen)
+        // {
+        //     count = oldLen;  // The count truncated by the old length
+        // }
+
+        if (count > 0)
         {
             // Recover overlapping content when resizing
 
@@ -58,7 +69,7 @@ void Foam::List<T>::resize_copy(const label count, const label len)
             // Can dispatch with
             // - std::execution::par_unseq
             // - std::execution::unseq
-            std::move(old, (old + overlap), this->v_);
+            std::move(old, (old + count), this->v_);
 
             ListPolicy::deallocate(old, oldLen);
         }
@@ -91,8 +102,6 @@ void Foam::List<T>::resize_copy(const label count, const label len)
 
 template<class T>
 Foam::List<T>::List(const label len)
-:
-    UList<T>(nullptr, len)
 {
     if (FOAM_UNLIKELY(len < 0))
     {
@@ -103,15 +112,14 @@ Foam::List<T>::List(const label len)
 
     if (len > 0)
     {
-        doAlloc();
+        // resize_nocopy()
+        doAlloc(len);
     }
 }
 
 
 template<class T>
 Foam::List<T>::List(const label len, const T& val)
-:
-    UList<T>(nullptr, len)
 {
     if (FOAM_UNLIKELY(len < 0))
     {
@@ -122,7 +130,8 @@ Foam::List<T>::List(const label len, const T& val)
 
     if (len > 0)
     {
-        doAlloc();
+        // resize_fill()
+        doAlloc(len);
         UList<T>::operator=(val);
     }
 }
@@ -130,8 +139,6 @@ Foam::List<T>::List(const label len, const T& val)
 
 template<class T>
 Foam::List<T>::List(const label len, Foam::zero)
-:
-    UList<T>(nullptr, len)
 {
     if (FOAM_UNLIKELY(len < 0))
     {
@@ -142,7 +149,8 @@ Foam::List<T>::List(const label len, Foam::zero)
 
     if (len > 0)
     {
-        doAlloc();
+        // resize_fill()
+        doAlloc(len);
         UList<T>::operator=(Foam::zero{});
     }
 }
@@ -177,12 +185,10 @@ Foam::List<T>::List(Foam::one, Foam::zero)
 
 template<class T>
 Foam::List<T>::List(const UList<T>& list)
-:
-    UList<T>(nullptr, list.size_)
 {
-    if (this->size_ > 0)
+    if (!list.empty())
     {
-        doAlloc();
+        doAlloc(list.size());
         UList<T>::deepCopy(list);
     }
 }
@@ -190,12 +196,10 @@ Foam::List<T>::List(const UList<T>& list)
 
 template<class T>
 Foam::List<T>::List(const List<T>& list)
-:
-    UList<T>(nullptr, list.size_)
 {
-    if (this->size_ > 0)
+    if (!list.empty())
     {
-        doAlloc();
+        doAlloc(list.size());
         UList<T>::deepCopy(list);
     }
 }
@@ -203,19 +207,18 @@ Foam::List<T>::List(const List<T>& list)
 
 template<class T>
 Foam::List<T>::List(List<T>& list, bool reuse)
-:
-    UList<T>(nullptr, list.size_)
 {
     if (reuse)
     {
         // Steal content
         this->v_ = list.v_;
+        this->size_ = list.size_;
         list.v_ = nullptr;
         list.size_ = 0;
     }
-    else if (this->size_ > 0)
+    else if (!list.empty())
     {
-        doAlloc();
+        doAlloc(list.size());
         UList<T>::deepCopy(list);
     }
 }
@@ -223,11 +226,12 @@ Foam::List<T>::List(List<T>& list, bool reuse)
 
 template<class T>
 Foam::List<T>::List(const UList<T>& list, const labelUList& indices)
-:
-    UList<T>(nullptr, indices.size())
 {
-    doAlloc();
-    copyList(list, indices);  // <- deepCopy()
+    if (!indices.empty())
+    {
+        doAlloc(indices.size());
+        copyList(list, indices);  // <- deepCopy()
+    }
 }
 
 
@@ -238,11 +242,12 @@ Foam::List<T>::List
     const UList<T>& list,
     const FixedList<label,N>& indices
 )
-:
-    UList<T>(nullptr, indices.size())
 {
-    doAlloc();
-    copyList(list, indices);  // <- deepCopy()
+    // if (!FixedList::empty()) is always true
+    {
+        doAlloc(indices.size());
+        copyList(list, indices);  // <- deepCopy()
+    }
 }
 
 
@@ -255,24 +260,23 @@ Foam::List<T>::List(const FixedList<T, N>& list)
 
 
 template<class T>
-Foam::List<T>::List(const PtrList<T>& list)
-:
-    UList<T>(nullptr, list.size())
+Foam::List<T>::List(const UPtrList<T>& list)
 {
-    doAlloc();
-    copyList(list);
+    if (!list.empty())
+    {
+        doAlloc(list.size());
+        copyList(list);
+    }
 }
 
 
 template<class T>
 template<class Addr>
 Foam::List<T>::List(const IndirectListBase<T, Addr>& list)
-:
-    UList<T>(nullptr, list.size())
 {
-    if (this->size_ > 0)
+    if (!list.empty())
     {
-        doAlloc();
+        doAlloc(list.size());
         UList<T>::deepCopy(list);
     }
 }
@@ -285,21 +289,9 @@ Foam::List<T>::List(std::initializer_list<T> list)
 {}
 
 
-template<class T>
-Foam::List<T>::List(List<T>&& list) noexcept
-:
-    UList<T>(list.data(), list.size())
-{
-    list.size_ = 0;
-    list.v_ = nullptr;
-}
-
-
 template<class T>
 template<int SizeMin>
 Foam::List<T>::List(DynamicList<T, SizeMin>&& list)
-:
-    UList<T>()
 {
     transfer(list);
 }
@@ -327,7 +319,7 @@ void Foam::List<T>::resize(const label len, const T& val)
         return;
     }
 
-    this->resize_copy(oldLen, len);
+    resize_copy(oldLen, len);
 
     // Fill trailing part with new values
     if (oldLen < this->size_)
@@ -368,7 +360,7 @@ void Foam::List<T>::transfer(DynamicList<T, SizeMin>& list)
     // Shrink the allocated space to the number of elements used
     list.shrink_to_fit();
     transfer(static_cast<List<T>&>(list));
-    list.clearStorage();  // Deletion, capacity=0 etc.
+    list.setCapacity_unsafe(0);  // All contents moved
 }
 
 
@@ -382,9 +374,9 @@ void Foam::List<T>::operator=(const UList<T>& list)
         return;  // Self-assignment is a no-op
     }
 
-    reAlloc(list.size_);
+    resize_nocopy(list.size());
 
-    if (this->size_ > 0)
+    if (!list.empty())
     {
         UList<T>::deepCopy(list);
     }
@@ -399,9 +391,9 @@ void Foam::List<T>::operator=(const List<T>& list)
         return;  // Self-assignment is a no-op
     }
 
-    reAlloc(list.size_);
+    resize_nocopy(list.size());
 
-    if (this->size_ > 0)
+    if (!list.empty())
     {
         UList<T>::deepCopy(list);
     }
@@ -412,7 +404,7 @@ template<class T>
 template<unsigned N>
 void Foam::List<T>::operator=(const FixedList<T, N>& list)
 {
-    reAlloc(list.size());
+    resize_nocopy(list.size());
 
     std::copy(list.begin(), list.end(), this->v_);
 }
@@ -422,7 +414,7 @@ template<class T>
 template<class Addr>
 void Foam::List<T>::operator=(const IndirectListBase<T, Addr>& list)
 {
-    reAlloc(list.size());
+    resize_nocopy(list.size());
     UList<T>::deepCopy(list);
 }
 
@@ -430,8 +422,7 @@ void Foam::List<T>::operator=(const IndirectListBase<T, Addr>& list)
 template<class T>
 void Foam::List<T>::operator=(std::initializer_list<T> list)
 {
-    reAlloc(list.size());
-
+    resize_nocopy(list.size());
     std::copy(list.begin(), list.end(), this->v_);
 }
 
diff --git a/src/OpenFOAM/containers/Lists/List/List.H b/src/OpenFOAM/containers/Lists/List/List.H
index 1dcfc62ba4..eaaaaea6a9 100644
--- a/src/OpenFOAM/containers/Lists/List/List.H
+++ b/src/OpenFOAM/containers/Lists/List/List.H
@@ -56,7 +56,7 @@ template<class T> class List;
 template<class T, unsigned N> class FixedList;
 template<class T, int SizeMin> class DynamicList;
 
-template<class T> class PtrList;
+template<class T> class UPtrList;
 
 template<class T> Istream& operator>>(Istream& is, List<T>& list);
 
@@ -77,12 +77,8 @@ class List
 {
     // Private Member Functions
 
-        //- Allocate list storage
-        inline void doAlloc();
-
-        //- Reallocate list storage to the given size
-        //  Discards old storage (if any). Does not copy old contents
-        inline void reAlloc(const label len);
+        //- Allocate list storage. Assumes there is no existing content
+        inline void doAlloc(const label len);
 
         //- Copy all list contents. Uses operator[] on the input list
         template<class ListType>
@@ -98,8 +94,8 @@ class List
         template<class InputIterator>
         inline List
         (
-            InputIterator firstIter,
-            InputIterator lastIter,  // (unused)
+            InputIterator input,
+            InputIterator inputEnd,  // (unused)
             const label len
         );
 
@@ -111,17 +107,7 @@ class List
     // Methods as per DynamicList to simplify code maintenance
 
         //- Stub method for internal naming as per DynamicList
-        void setCapacity_nocopy(const label len) { resize_nocopy(len); }
-
-
-protected:
-
-    // Protected Member Functions
-
-        //- Low-level resizing (backend for resize).
-        //- Change allocation size of list, retaining the first count contents.
-        //  \note Only uses a limited number of internal checks.
-        void resize_copy(const label count, const label len);
+        void setCapacity_nocopy(label len) { resize_nocopy(len); }
 
 
 public:
@@ -185,8 +171,8 @@ public:
         template<unsigned N>
         explicit List(const FixedList<T, N>& list);
 
-        //- Construct as copy of PtrList<T>
-        explicit List(const PtrList<T>& list);
+        //- Construct as copy of UPtrList<T> content
+        explicit List(const UPtrList<T>& list);
 
         //- Construct as copy of IndirectList contents
         template<class Addr>
@@ -196,7 +182,7 @@ public:
         List(std::initializer_list<T> list);
 
         //- Move construct from List
-        List(List<T>&& list) noexcept;
+        inline List(List<T>&& list) noexcept;
 
         //- Move construct from DynamicList
         template<int SizeMin>
@@ -227,6 +213,10 @@ public:
         //- Adjust allocated size of list and set val for \em new elements
         void resize(const label len, const T& val);
 
+        //- Change allocated size of list, retaining the first count elements.
+        //  \note Only uses a limited number of internal checks.
+        void resize_copy(label count, const label len);
+
         //- Adjust allocated size of list and set val for \em all elements
         inline void resize_fill(const label len, const T& val);
 
@@ -234,14 +224,9 @@ public:
         //  retaining old content.
         //  If no reallocation is required, the contents remain untouched.
         //  Otherwise the contents will be uninitialized.
+        //  Shorthand for \c resize(0, len)
         inline void resize_nocopy(const label len);
 
-        //- Alias for resize()
-        void setSize(const label n) { this->resize(n); }
-
-        //- Alias for resize()
-        void setSize(const label n, const T& val) { this->resize(n, val); }
-
 
     // Edit
 
@@ -404,6 +389,12 @@ public:
         //- Same as push_uniq()
         FOAM_DEPRECATED_FOR(2022-10, "push_uniq()")
         label appendUniq(const T& val) { return this->push_uniq(val); }
+
+        //- Alias for resize()
+        void setSize(label n) { this->resize(n); }
+
+        //- Alias for resize()
+        void setSize(label n, const T& val) { this->resize(n, val); }
 };
 
 
diff --git a/src/OpenFOAM/containers/Lists/List/ListI.H b/src/OpenFOAM/containers/Lists/List/ListI.H
index e37a2ea271..591e6a2e60 100644
--- a/src/OpenFOAM/containers/Lists/List/ListI.H
+++ b/src/OpenFOAM/containers/Lists/List/ListI.H
@@ -29,24 +29,13 @@ License
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
 template<class T>
-inline void Foam::List<T>::doAlloc()
+inline void Foam::List<T>::doAlloc(const label len)
 {
-    if (this->size_ > 0)
+    if (len > 0)
     {
         // With sign-check to avoid spurious -Walloc-size-larger-than
-        this->v_ = ListPolicy::allocate<T>(this->size_);
-    }
-}
-
-
-template<class T>
-inline void Foam::List<T>::reAlloc(const label len)
-{
-    if (this->size_ != len)
-    {
-        clear();
+        this->v_ = ListPolicy::allocate<T>(len);
         this->size_ = len;
-        doAlloc();
     }
 }
 
@@ -95,23 +84,24 @@ template<class T>
 template<class InputIterator>
 inline Foam::List<T>::List
 (
-    InputIterator firstIter,
-    InputIterator lastIter,  // (unused)
+    InputIterator input,
+    InputIterator inputEnd,  // (unused)
     const label len
 )
-:
-    UList<T>(nullptr, len)
 {
     if (len > 0)
     {
-        doAlloc();
+        doAlloc(len);
 
         // Like std::copy() or std::copy_n()
         // but without any requirements on the iterator category
 
-        for (label i = 0; i < len; (void)++i, (void)++firstIter)
+        auto iter = this->v_;
+        const auto endIter = (iter + len);
+
+        for (; iter != endIter; (void)++iter, (void)++input)
         {
-            this->v_[i] = *firstIter;
+            *iter = *input;
         }
     }
 }
@@ -124,6 +114,17 @@ inline constexpr Foam::List<T>::List() noexcept
 {}
 
 
+template<class T>
+inline Foam::List<T>::List(List<T>&& list) noexcept
+:
+    UList<T>(list.data(), list.size())
+{
+    // Stole content
+    list.size_ = 0;
+    list.v_ = nullptr;
+}
+
+
 template<class T>
 inline Foam::autoPtr<Foam::List<T>> Foam::List<T>::clone() const
 {
@@ -158,7 +159,7 @@ inline void Foam::List<T>::resize(const label len)
 {
     if (this->size_ != len)
     {
-        this->resize_copy(this->size_, len);
+        resize_copy(this->size_, len);
     }
 }
 
@@ -166,7 +167,7 @@ inline void Foam::List<T>::resize(const label len)
 template<class T>
 inline void Foam::List<T>::resize_fill(const label len, const T& val)
 {
-    this->reAlloc(len);
+    resize_nocopy(len);
     UList<T>::operator=(val);
 }
 
@@ -174,7 +175,12 @@ inline void Foam::List<T>::resize_fill(const label len, const T& val)
 template<class T>
 inline void Foam::List<T>::resize_nocopy(const label len)
 {
-    this->reAlloc(len);
+    // Same as resize_copy(0, len);
+    if (this->size_ != len)
+    {
+        clear();
+        doAlloc(len);
+    }
 }
 
 
diff --git a/src/OpenFOAM/containers/Lists/List/UList.H b/src/OpenFOAM/containers/Lists/List/UList.H
index bdac7f55a3..7614575dc8 100644
--- a/src/OpenFOAM/containers/Lists/List/UList.H
+++ b/src/OpenFOAM/containers/Lists/List/UList.H
@@ -628,6 +628,15 @@ public:
             return false;
         }
 
+        //- Return a string_view of the charList. Content is non-modifiable.
+        template<class TypeT = T>
+        std::enable_if_t
+        <std::is_same_v<char, std::remove_cv_t<TypeT>>, std::string_view>
+        inline view() const
+        {
+            return std::string_view(v_, size_);
+        }
+
 
     // Hashing
 
diff --git a/src/OpenFOAM/db/IOstreams/Fstreams/IFstream.C b/src/OpenFOAM/db/IOstreams/Fstreams/IFstream.C
index 664328400c..ae7f9c38cf 100644
--- a/src/OpenFOAM/db/IOstreams/Fstreams/IFstream.C
+++ b/src/OpenFOAM/db/IOstreams/Fstreams/IFstream.C
@@ -275,9 +275,11 @@ const std::istream& Foam::IFstream::stdStream() const
 
 void Foam::IFstream::rewind()
 {
+    Istream::rewind();  // Drop any putback
+    lineNumber_ = 1;    // Reset line number
+
     if (IOstreamOption::COMPRESSED == ifstreamPointer::whichCompression())
     {
-        lineNumber_ = 1;  // Reset line number
         ifstreamPointer::reopen_gz(this->name());
         setState(ifstreamPointer::get()->rdstate());
     }
diff --git a/src/OpenFOAM/db/IOstreams/IOstreams/Istream.C b/src/OpenFOAM/db/IOstreams/IOstreams/Istream.C
index 0c6b4cd824..1a38dd9c60 100644
--- a/src/OpenFOAM/db/IOstreams/IOstreams/Istream.C
+++ b/src/OpenFOAM/db/IOstreams/IOstreams/Istream.C
@@ -218,6 +218,12 @@ char Foam::Istream::readEndList(const char* funcName)
 }
 
 
+void Foam::Istream::rewind()
+{
+    putBackClear();  // Drop any putback
+}
+
+
 Foam::Istream& Foam::Istream::operator()() const
 {
     if (!good())
diff --git a/src/OpenFOAM/db/IOstreams/IOstreams/Istream.H b/src/OpenFOAM/db/IOstreams/IOstreams/Istream.H
index eecf7aa0a4..2849b5c2f6 100644
--- a/src/OpenFOAM/db/IOstreams/IOstreams/Istream.H
+++ b/src/OpenFOAM/db/IOstreams/IOstreams/Istream.H
@@ -175,7 +175,8 @@ public:
             //- operation.
             virtual Istream& readRaw(char* data, std::streamsize count) = 0;
 
-            //- Rewind the stream so that it may be read again
+            //- Rewind the stream so that it may be read again.
+            //  The base implementation clears any putback.
             virtual void rewind() = 0;
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
index c9444e6735..589e757e2d 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
@@ -563,6 +563,8 @@ Foam::label Foam::UIPstreamBase::remaining() const noexcept
 
 void Foam::UIPstreamBase::rewind()
 {
+    Istream::rewind();  // Drop any putback
+
     recvBufPos_ = 0;  // Assume the entire buffer is for us to read from
     setOpened();
     setGood();
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.H
new file mode 100644
index 0000000000..6c7db54231
--- /dev/null
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.H
@@ -0,0 +1,267 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Class
+    Foam::UPstream::File
+
+Description
+    An opaque wrapper for MPI_File methods
+    without any \c <mpi.h> header dependency.
+
+Note
+    Not included as part of UPstream.H - only include locally as required
+
+SourceFiles
+    UPstreamFile.txx
+
+\*---------------------------------------------------------------------------*/
+
+#ifndef Foam_UPstreamFile_H
+#define Foam_UPstreamFile_H
+
+#include "fileName.H"
+#include "UPstream.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+
+/*---------------------------------------------------------------------------*\
+                       Class UPstream::File Declaration
+\*---------------------------------------------------------------------------*/
+
+class UPstream::File
+{
+    // Forward Declaration
+    class Impl;
+
+
+    // Private Data
+
+        //- Implementation wrapper of MPI_File etc
+        std::unique_ptr<Impl> file_;
+
+
+protected:
+
+    // Protected Method Functions
+
+        //- MPI_File_write [non-collective] : write data
+        bool write_data
+        (
+            //! Source buffer
+            const void* buffer,
+            //! The data count - number of elements
+            std::streamsize count,
+            const UPstream::dataTypes dataTypeId
+        );
+
+        //- MPI_File_write_at [non-collective] : write data at specified offset
+        bool write_data_at
+        (
+            //! The offset - number of 'view' elements (default: byte)
+            std::streamsize offset,
+            //! Source buffer
+            const void* buffer,
+            //! The data count - number of elements
+            std::streamsize count,
+            const UPstream::dataTypes dataTypeId
+        );
+
+        //- MPI_File_write_all [collective] : write data
+        bool write_data_all
+        (
+            //! Source buffer
+            const void* buffer,
+            //! The data count - number of elements
+            std::streamsize count,
+            const UPstream::dataTypes dataTypeId
+        );
+
+        //- MPI_File_write_at_all [collective] :
+        //- write data at specified offset
+        bool write_data_at_all
+        (
+            //! The offset - number of 'view' elements (default: byte)
+            std::streamsize offset,
+            //! Source buffer
+            const void* buffer,
+            //! The data count - number of elements
+            std::streamsize count,
+            const UPstream::dataTypes dataTypeId
+        );
+
+
+public:
+
+    // Generated Methods
+
+        //- No copy construct
+        File(const File&&) = delete;
+
+        //- Move construct
+        File(File&&) noexcept;
+
+        //- No copy assignment
+        File& operator=(const File&) = delete;
+
+        //- Move assignment
+        File& operator=(File&&) noexcept;
+
+
+    // Constructors
+
+        //- Default construct
+        File();
+
+        //- Destructor. Non-default in header (incomplete types)
+        ~File();
+
+
+    // Static Member Functions
+
+        //- True if MPI/IO appears to be supported
+        static bool supported();
+
+
+    // Member Functions
+
+    // Access
+
+        //- The name of the open stream
+        const fileName& name() const;
+
+        //- True if allocated and open has been called
+        bool is_open() const;
+
+
+    // Basics
+
+        //- MPI_File_open [collective] :
+        //- open file in write-only mode, no-append
+        bool open_write
+        (
+            //! The OpenFOAM communicator index
+            const int communicator,
+            //! Full file path (parent directory must exist before calling)
+            const fileName& pathname,
+            //! Simulated atomic file handling
+            IOstreamOption::atomicType = IOstreamOption::NON_ATOMIC
+        );
+
+        //- MPI_File_close [collective]
+        bool close();
+
+        //- Set the (output) file size [collective]
+        bool set_size(std::streamsize num_bytes);
+
+
+    // Writing
+
+        //- MPI_File_write [non-collective] : write data.
+        //  A no-op and return true if content is empty
+        inline bool write(std::string_view sv);
+
+        //- MPI_File_write [non-collective] : write data.
+        //  A no-op and return true if buffer is nullptr or count is zero
+        template<class Type>
+        inline bool write
+        (
+            //! The content
+            const Type* buffer,
+            //! The data count - number of elements
+            std::streamsize count
+        );
+
+        //- MPI_File_write_at [non-collective] : write data at specified offset.
+        //  A no-op and return true if content is empty
+        inline bool write_at(std::streamsize offset, std::string_view sv);
+
+        //- MPI_File_write_at [non-collective] : write data at specified offset.
+        //  A no-op and return true if buffer is nullptr or count is zero
+        template<class Type>
+        inline bool write_at
+        (
+            //! The offset within the file - number of 'view' elements
+            std::streamsize offset,
+            //! The content
+            const Type* buffer,
+            //! The data count - number of elements
+            std::streamsize count
+        );
+
+        //- MPI_File_write_all [collective] : write data
+        inline bool write_all(std::string_view sv);
+
+        //- MPI_File_write_all [collective] : write data
+        template<class Type>
+        inline bool write_all
+        (
+            //! The content
+            const Type* buffer,
+            //! The data count - number of elements
+            std::streamsize count
+        );
+
+        //- MPI_File_write_at_all [collective] :
+        //- write data at specified offset
+        inline bool write_at_all
+        (
+            //! The offset within the file - number of 'view' elements
+            std::streamsize offset,
+            //! The content
+            std::string_view sv
+        );
+
+        //- MPI_File_write_at_all [collective] :
+        //- write data at specified offset
+        template<class Type>
+        inline bool write_at_all
+        (
+            //! The offset within the file - number of 'view' elements
+            std::streamsize offset,
+            //! The content
+            const Type* buffer,
+            //! The data count - number of elements
+            std::streamsize count
+        );
+};
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+} // End namespace Foam
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#include "UPstreamFile.txx"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+#endif
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.txx b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.txx
new file mode 100644
index 0000000000..d41f029c61
--- /dev/null
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamFile.txx
@@ -0,0 +1,220 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+inline bool Foam::UPstream::File::write
+(
+    std::string_view sv
+)
+{
+    if (sv.empty())
+    {
+        // no-op for no content
+        return true;
+    }
+
+    return this->write_data
+    (
+        sv.data(), sv.size(),
+        UPstream::dataTypes::type_byte
+    );
+}
+
+
+template<class Type>
+bool Foam::UPstream::File::write
+(
+    const Type* buffer,
+    std::streamsize count
+)
+{
+    if constexpr (!is_contiguous_v<Type>)
+    {
+        FatalErrorInFunction
+            << "Only contiguous data can be supported!"
+            << Foam::abort(FatalError);
+        return false;
+    }
+    else if (buffer && count > 1)
+    {
+        // Use element or component type (or byte-wise) for data type
+        return this->write_data
+        (
+            buffer,  // The data or cmpt pointer
+            UPstream_dataType<Type>::size(count),
+            UPstream_dataType<Type>::datatype_id
+        );
+    }
+    else
+    {
+        // no-op for no content
+        return true;
+    }
+}
+
+
+inline bool Foam::UPstream::File::write_at
+(
+    std::streamsize offset,
+    std::string_view sv
+)
+{
+    if (sv.empty())
+    {
+        // no-op for no content
+        return true;
+    }
+
+    return this->write_data_at
+    (
+        offset,
+        sv.data(), sv.size(),
+        UPstream::dataTypes::type_byte
+    );
+}
+
+
+template<class Type>
+bool Foam::UPstream::File::write_at
+(
+    std::streamsize offset,
+    const Type* buffer,
+    std::streamsize count
+)
+{
+    if constexpr (!is_contiguous_v<Type>)
+    {
+        FatalErrorInFunction
+            << "Only contiguous data can be supported!"
+            << Foam::abort(FatalError);
+        return false;
+    }
+    else if (buffer && count > 1)
+    {
+        // Use element or component type (or byte-wise) for data type
+        return this->write_data_at
+        (
+            offset,
+            buffer,  // The data or cmpt pointer
+            UPstream_dataType<Type>::size(count),
+            UPstream_dataType<Type>::datatype_id
+        );
+    }
+    else
+    {
+        // no-op for no content
+        return true;
+    }
+}
+
+
+inline bool Foam::UPstream::File::write_all
+(
+    std::string_view sv
+)
+{
+    return this->write_data_all
+    (
+        sv.data(), sv.size(),
+        UPstream::dataTypes::type_byte
+    );
+}
+
+
+template<class Type>
+bool Foam::UPstream::File::write_all
+(
+    const Type* buffer,
+    std::streamsize count
+)
+{
+    if constexpr (!is_contiguous_v<Type>)
+    {
+        FatalErrorInFunction
+            << "Only contiguous data can be supported!"
+            << Foam::abort(FatalError);
+        return false;
+    }
+    else
+    {
+        // Use element or component type (or byte-wise) for data type
+        return this->write_data_all
+        (
+            buffer,  // The data or cmpt pointer
+            UPstream_dataType<Type>::size(count),
+            UPstream_dataType<Type>::datatype_id
+        );
+    }
+}
+
+
+inline bool Foam::UPstream::File::write_at_all
+(
+    std::streamsize offset,
+    std::string_view sv
+)
+{
+    return this->write_data_at_all
+    (
+        offset,
+        sv.data(), sv.size(),
+        UPstream::dataTypes::type_byte
+    );
+}
+
+
+template<class Type>
+bool Foam::UPstream::File::write_at_all
+(
+    std::streamsize offset,
+    const Type* buffer,
+    std::streamsize count
+)
+{
+    if constexpr (!is_contiguous_v<Type>)
+    {
+        FatalErrorInFunction
+            << "Only contiguous data can be supported!"
+            << Foam::abort(FatalError);
+        return false;
+    }
+    else
+    {
+        // Use element or component type (or byte-wise) for data type
+        return this->write_data_at_all
+        (
+            offset,
+            buffer,  // The data or cmpt pointer
+            UPstream_dataType<Type>::size(count),
+            UPstream_dataType<Type>::datatype_id
+        );
+    }
+}
+
+
+// ************************************************************************* //
diff --git a/src/OpenFOAM/db/IOstreams/Sstreams/ISstream.C b/src/OpenFOAM/db/IOstreams/Sstreams/ISstream.C
index 48d63eb49a..7f110596af 100644
--- a/src/OpenFOAM/db/IOstreams/Sstreams/ISstream.C
+++ b/src/OpenFOAM/db/IOstreams/Sstreams/ISstream.C
@@ -1084,6 +1084,7 @@ bool Foam::ISstream::endRawRead()
 
 void Foam::ISstream::rewind()
 {
+    Istream::rewind();    // Drop any putback
     lineNumber_ = 1;      // Reset line number
 
     stdStream().clear();  // Clear the iostate error state flags
diff --git a/src/OpenFOAM/db/IOstreams/Tstreams/ITstream.H b/src/OpenFOAM/db/IOstreams/Tstreams/ITstream.H
index a4055ef452..454e371df7 100644
--- a/src/OpenFOAM/db/IOstreams/Tstreams/ITstream.H
+++ b/src/OpenFOAM/db/IOstreams/Tstreams/ITstream.H
@@ -531,7 +531,11 @@ public:
         virtual bool endRawRead() override { return false; }
 
         //- Rewind the stream so that it may be read again. Same as seek(0)
-        virtual void rewind() override { ITstream::seek(0); }
+        virtual void rewind() override
+        {
+            Istream::rewind();
+            ITstream::seek(0);
+        }
 
 
     // Output
diff --git a/src/OpenFOAM/db/IOstreams/memory/ICharStream.H b/src/OpenFOAM/db/IOstreams/memory/ICharStream.H
index 8496a80376..2a1dd44b1e 100644
--- a/src/OpenFOAM/db/IOstreams/memory/ICharStream.H
+++ b/src/OpenFOAM/db/IOstreams/memory/ICharStream.H
@@ -357,6 +357,7 @@ public:
         //- Rewind the stream, clearing any old errors
         virtual void rewind() override
         {
+            Istream::rewind();
             stream_.rewind();
             syncState();
         }
diff --git a/src/OpenFOAM/db/IOstreams/memory/ISpanStream.H b/src/OpenFOAM/db/IOstreams/memory/ISpanStream.H
index 8b5cc19700..ae73dbf42c 100644
--- a/src/OpenFOAM/db/IOstreams/memory/ISpanStream.H
+++ b/src/OpenFOAM/db/IOstreams/memory/ISpanStream.H
@@ -419,6 +419,7 @@ public:
         //- Rewind the stream, clearing any old errors
         virtual void rewind() override
         {
+            Istream::rewind();
             stream_.rewind();
             syncState();
         }
diff --git a/src/OpenFOAM/db/IOstreams/memory/OCharStream.H b/src/OpenFOAM/db/IOstreams/memory/OCharStream.H
index 274e690d15..c5e2b643e1 100644
--- a/src/OpenFOAM/db/IOstreams/memory/OCharStream.H
+++ b/src/OpenFOAM/db/IOstreams/memory/OCharStream.H
@@ -31,8 +31,7 @@ Description
     Similar to OStringStream but with a List for its storage instead of
     as string to allow reuse of List contents without copying.
 
-    The default initial size is 512-bytes and uses size doubling.
-    After construction can use the reserve() method to adjust this.
+    Internally imposes a 512 byte min-size and uses capacity doubling.
 
 See Also
     Foam::ICharStream
@@ -131,6 +130,25 @@ public:
             buffer_type::reserve(n);
         }
 
+        //- Reserve output space for at least this amount.
+        //- Does not apply min-size or capacity doubling etc.
+        void reserve_exact(std::streamsize n)
+        {
+            buffer_type::reserve_exact(n);
+        }
+
+        //- Increase (reserve) space for another \c count entries
+        void extend(std::streamsize count)
+        {
+            buffer_type::extend(count);
+        }
+
+        //- Increase (reserve) space for another \c count entries
+        void extend_exact(std::streamsize count)
+        {
+            buffer_type::extend_exact(count);
+        }
+
         //- Rewind the stream, clearing any old errors
         void rewind()
         {
@@ -209,6 +227,64 @@ public:
 
         //- Information about stream
         void print(Ostream& os) const { debug_info(os); os << '\n'; }
+
+
+    // Extra/Convenience Methods
+
+        //- Append a single character to the end
+        void push_back(char c) { stream_type::put(c); }
+
+        //- Rewind the end by 1 or more elements
+        void pop_back(int n = 1) { buffer_type::pop_back(n); }
+
+        //- Append repeated character content
+        void append(std::streamsize count, char c)
+        {
+            if (count > 0)
+            {
+                buffer_type::extend(count);
+                while (count-- > 0)
+                {
+                    stream_type::put(c);
+                }
+            }
+        }
+
+        //- Append character content - like a plain write()
+        void append(const char* data, std::streamsize count)
+        {
+            if (data && count > 0)
+            {
+                buffer_type::extend(count);
+                write(data, count);
+            }
+        }
+
+        //- Overwrite a single character
+        void overwrite(std::streampos pos, char c)
+        {
+            buffer_type::overwrite(pos, c);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite
+        (
+            std::streampos pos,
+            const char* data,
+            std::streamsize count
+        )
+        {
+            buffer_type::overwrite(pos, data, count);
+        }
+
+        //- The output data (start of output characters)
+        const char* cdata_bytes() const { return buffer_type::data_bytes(); }
+
+        //- The output data (start of output characters)
+        char* data_bytes() { return buffer_type::data_bytes(); }
+
+        //- The current number of output characters
+        std::streamsize size_bytes() const { return buffer_type::size_bytes(); }
 };
 
 
@@ -287,6 +363,15 @@ public:
         //- Reserve output space for at least this amount
         void reserve(std::streamsize n) { stream_.reserve(n); }
 
+        //- Reserve output space for at least this amount.
+        //- Does not apply min-size or capacity doubling etc.
+        void reserve_exact(std::streamsize n) { stream_.reserve_exact(n); }
+
+        //- Increase (reserve) space for another \c n entries
+        void extend(std::streamsize n) { stream_.extend(n); }
+
+        //- Increase (reserve) space for another \c n entries
+        void extend_exact(std::streamsize n) { stream_.extend_exact(n); }
 
         //- A string_view of buffer contents
         auto view() const { return stream_.view(); }
@@ -350,6 +435,56 @@ public:
         }
 
 
+    // Extra/Convenience Methods
+
+        //- Append a single character to the end
+        void push_back(char c) { stream_.push_back(c); }
+
+        //- Rewind the end by 1 or more elements
+        void pop_back(int n = 1) { stream_.pop_back(n); }
+
+        //- Append repeated character content
+        void append(std::streamsize count, char c)
+        {
+            stream_.append(count, c);
+        }
+
+        //- Append character content
+        void append(const char* data, std::streamsize count)
+        {
+            stream_.append(data, count);
+        }
+
+        //- Append character content
+        void append(std::string_view sv)
+        {
+            stream_.append(sv.data(), sv.size());
+        }
+
+        //- Overwrite a single character
+        void overwrite(std::streampos pos, char c)
+        {
+            stream_.overwrite(pos, c);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite
+        (
+            std::streampos pos,
+            const char* data,
+            std::streamsize count
+        )
+        {
+            stream_.overwrite(pos, data, count);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite(std::streampos pos, std::string_view sv)
+        {
+            stream_.overwrite(pos, sv.data(), sv.size());
+        }
+
+
     // Housekeeping
 
         //- Block size was used in OpenFOAM-v2306 and earlier
diff --git a/src/OpenFOAM/db/IOstreams/memory/OSpanStream.H b/src/OpenFOAM/db/IOstreams/memory/OSpanStream.H
index ba8c34710e..e1611a7502 100644
--- a/src/OpenFOAM/db/IOstreams/memory/OSpanStream.H
+++ b/src/OpenFOAM/db/IOstreams/memory/OSpanStream.H
@@ -226,6 +226,23 @@ public:
 
     // Extra/Convenience Methods
 
+        //- Overwrite a single character
+        void overwrite(std::streampos pos, char c)
+        {
+            buffer_type::overwrite(pos, c);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite
+        (
+            std::streampos pos,
+            const char* data,
+            std::streamsize count
+        )
+        {
+            buffer_type::overwrite(pos, data, count);
+        }
+
         //- The output data (start of output characters)
         const char* cdata_bytes() const { return buffer_type::data_bytes(); }
 
@@ -234,7 +251,6 @@ public:
 
         //- The current number of output characters
         std::streamsize size_bytes() const { return buffer_type::size_bytes(); }
-
 };
 
 
@@ -382,6 +398,47 @@ public:
             stream_.debug_info(os);
             os << '\n';
         }
+
+
+    // Extra/Convenience Methods
+
+        //- Append character content - same as writeRaw()
+        void append(const char* data, std::streamsize count)
+        {
+            if (data && count > 0)
+            {
+                writeRaw(data, count);
+            }
+        }
+
+        //- Append character content - same as writeRaw()
+        void append(std::string_view sv)
+        {
+            append(sv.data(), sv.size());
+        }
+
+        //- Overwrite a single character
+        void overwrite(std::streampos pos, char c)
+        {
+            stream_.overwrite(pos, c);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite
+        (
+            std::streampos pos,
+            const char* data,
+            std::streamsize count
+        )
+        {
+            stream_.overwrite(pos, data, count);
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite(std::streampos pos, std::string_view sv)
+        {
+            stream_.overwrite(pos, sv.data(), sv.size());
+        }
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/memory/memoryStreamBuffer.H b/src/OpenFOAM/db/IOstreams/memory/memoryStreamBuffer.H
index caf3fa1bb3..4fc8461dc6 100644
--- a/src/OpenFOAM/db/IOstreams/memory/memoryStreamBuffer.H
+++ b/src/OpenFOAM/db/IOstreams/memory/memoryStreamBuffer.H
@@ -468,6 +468,43 @@ public:
             }
         }
 
+        //- Decrease the put area by 1 or more elements
+        void pop_back(int n = 1)
+        {
+            for (; (n > 0 && (pbase() < pptr())); --n)
+            {
+                pbump(-1);
+            }
+        }
+
+        //- Overwrite a sub-slice with character content
+        void overwrite
+        (
+            std::streampos pos,
+            const char* data,
+            std::streamsize count
+        )
+        {
+            if (data && (count > 0) && in_range(pos))
+            {
+                // Restrict to intersection with current content
+                if (span_tellp() <= std::streampos(pos+count))
+                {
+                    count = (span_tellp() - pos);
+                }
+                std::copy(data, (data+count), (data_bytes()+pos));
+            }
+        }
+
+        //- Overwrite a single character
+        void overwrite(std::streampos pos, char c)
+        {
+            if (c && in_range(pos))
+            {
+                *(data_bytes()+pos) = c;
+            }
+        }
+
         //- Some information about the output buffer position/capacity
         void info(Ostream& os) const
         {
@@ -500,8 +537,8 @@ protected:
     {
         if (c != traits_type::eof())
         {
-            // Need more space?
-            reserve(1 + span_tellp());
+            // Needed more space?
+            extend(1);
 
             *(pptr()) = c;
             pbump(1);
@@ -513,7 +550,7 @@ protected:
     virtual std::streamsize xsputn(const char* s, std::streamsize n)
     {
         // Enough space so that appends work without problem
-        reserve(n + span_tellp());
+        extend(n);
 
         std::streamsize count = 0;
         while (count < n && pptr() < epptr())
@@ -530,10 +567,14 @@ public:
 
     // Constructors
 
+        //- Default construct - no initial reserved number of bytes.
+        out_dynamic()
+        {
+            sync_pbuffer();
+        }
+
         //- Default construct with initial reserved number of bytes.
-        //  The value of 512 is a bit arbitrary, but consistent with
-        //  std::stringstream
-        out_dynamic(size_t nbytes = 512)
+        out_dynamic(size_t nbytes)
         :
             storage_(label(nbytes))
         {
@@ -560,40 +601,115 @@ public:
 
     // Member Functions
 
-        //- Increment capacity (if needed) and adjust buffer pointers
+        //- Normal lower capacity limit.
+        //  512 bytes is a bit arbitrary but consistent with std::stringstream
+        static constexpr label min_size() noexcept { return 512; }
+
+        //- The largest storage size
+        static constexpr label max_size() noexcept
+        {
+            return UList<char>::max_size();
+        }
+
+        //- The 1/2 of max_size() - rounded to power-of-two
+        static constexpr label max_size_2() noexcept
+        {
+            return (1+max_size()/2);
+        }
+
+        //- The 1/4 of max_size() - rounded to power-of-two
+        static constexpr label max_size_4() noexcept
+        {
+            return (max_size_2()/2);
+        }
+
+        //- Sync put buffer pointers to agree with list dimensions.
+        //  Sets put pointer to the begin (rewind).
+        void sync_pbuffer()
+        {
+            resetp(storage_.data(), storage_.size());
+        }
+
+        //- Increase capacity (if needed) and adjust buffer pointers.
+        //- Applies a min-size and capacity doubling.
         void reserve(const std::streamsize len)
         {
             if (storage_.size() < len)
             {
-                const auto cur = span_tellp();   // Current location
+                const auto cur = span_tellp();  // Output position
 
-                label newCapacity = 512;
+                label size = min_size();
 
-                if (newCapacity < len)
+                if (size < len)
                 {
-                    // Increase capacity (doubling)
-                    newCapacity =
-                        Foam::max(label(len), label(2*storage_.size()));
-                    // ratio=1.5:
-                    // Foam::max(label(len), label((storage_.size()/2)*3));
+                    // Increase capacity, but grow more slowly at the largest
+                    // sizes. Expect a buffer of char to approach max_size()
+                    // more commonly than buffers of other data types.
+
+                    if (storage_.size() <= max_size_4())
+                    {
+                        // (0 < capacity <= 0.25) : fast growth (2)
+                        size = label(2*storage_.size());
+                    }
+                    else if (storage_.size() <= max_size_2())
+                    {
+                        // (0.25 < capacity <= 0.5) : slower growth (1.5)
+                        size = label((storage_.size()/2)*3);
+                    }
+                    else if (storage_.size() <= (max_size_2() + max_size_4()))
+                    {
+                        // (0.5 < capacity <= 0.75) : very slow growth (1.25)
+                        size = label((storage_.size()/4)*5);
+                    }
+                    else
+                    {
+                        // Already very large - use max (or just len?)
+                        size = max_size();
+                    }
+
+                    // max(size, len)
+                    if (size < len)
+                    {
+                        size = len;
+                    }
                 }
 
-                // Info<<"request:" << len
-                //     << " cur cap:" << storage_.size()
-                //     << " new cap:" << newCapacity
+                // std::cerr
+                //     <<"request:" << len
+                //     << " old cap:" << storage_.size()
+                //     << " new cap:" << size
                 //     << " pos:" << cur << endl;
 
-                storage_.resize(newCapacity);
+                storage_.resize_copy(cur, size);
                 sync_pbuffer();
                 pbump(cur);
             }
         }
 
-        //- Sync put buffer pointers to agree with list dimensions
-        //  Sets put pointer to the begin (rewind).
-        void sync_pbuffer()
+        //- Increase capacity for at least this size.
+        //- Does not apply min-size or capacity doubling etc.
+        void reserve_exact(const std::streamsize len)
         {
-            resetp(storage_.data(), storage_.size());
+            if (storage_.size() < len)
+            {
+                const auto cur = span_tellp();  // Output position
+
+                storage_.resize_copy(cur, len);
+                sync_pbuffer();
+                pbump(cur);
+            }
+        }
+
+        //- Increase (reserve) space for another \c count entries
+        void extend(std::streamsize count)
+        {
+            reserve(span_tellp() + count);
+        }
+
+        //- Increase (reserve) space for another \c count entries
+        void extend_exact(std::streamsize count)
+        {
+            reserve_exact(span_tellp() + count);
         }
 
         //- Clear storage
diff --git a/src/OpenFOAM/fields/Fields/DynamicField/DynamicField.H b/src/OpenFOAM/fields/Fields/DynamicField/DynamicField.H
index 5679ca5b9c..3b5f3e5321 100644
--- a/src/OpenFOAM/fields/Fields/DynamicField/DynamicField.H
+++ b/src/OpenFOAM/fields/Fields/DynamicField/DynamicField.H
@@ -79,19 +79,14 @@ class DynamicField
         template<class ListType>
         inline void doAssignDynList(const ListType& list);
 
-        //- Alter the size of the underlying storage
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doCapacity(const bool nocopy, const label len);
+        //- Alter the size of the underlying storage,
+        //- retaining the first count elements.
+        inline void doCapacity_copy(label count, const label len);
 
-        //- Reserve allocation space for at least this size.
+        //- Reserve allocation space for at least this size,
+        //- retaining the first count elements.
         //  Never shrinks the allocated size, use setCapacity() for that.
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doReserve(const bool nocopy, const label len);
-
-        //- Reserve allocation space for at least this size.
-        //  Never shrinks the allocated size, use setCapacity() for that.
-        //  The 'nocopy' option will not attempt to recover old content
-        inline void doResize(const bool nocopy, const label len);
+        inline void doReserve_copy(label count, const label len);
 
 
 public:
@@ -246,6 +241,10 @@ public:
         //- Alter addressable size and fill \em new entries with constant value
         inline void resize(const label len, const T& val);
 
+        //- Alter addressable size, retaining the first count contents.
+        //  \note Only uses a limited number of internal checks.
+        inline void resize_copy(label count, const label len);
+
         //- Alter addressable size and set val for \em all addressed entries
         inline void resize_fill(const label len, const T& val);
 
diff --git a/src/OpenFOAM/fields/Fields/DynamicField/DynamicFieldI.H b/src/OpenFOAM/fields/Fields/DynamicField/DynamicFieldI.H
index 67ced92b34..602d4aae99 100644
--- a/src/OpenFOAM/fields/Fields/DynamicField/DynamicFieldI.H
+++ b/src/OpenFOAM/fields/Fields/DynamicField/DynamicFieldI.H
@@ -52,9 +52,9 @@ inline void Foam::DynamicField<T, SizeMin>::doAssignDynList
 
 
 template<class T, int SizeMin>
-inline void Foam::DynamicField<T, SizeMin>::doCapacity
+inline void Foam::DynamicField<T, SizeMin>::doCapacity_copy
 (
-    const bool nocopy,
+    label count,
     const label newCapacity
 )
 {
@@ -66,16 +66,22 @@ inline void Foam::DynamicField<T, SizeMin>::doCapacity
     // Addressable length, possibly truncated by new capacity
     const label currLen = Foam::min(List<T>::size(), newCapacity);
 
+    // The count truncated by the new addressable range
+    if (count > currLen)
+    {
+        count = currLen;
+    }
+
     // Consistent allocated sizing
     List<T>::setAddressableSize(capacity_);
 
-    if (nocopy)
+    if (count > 0)
     {
-        List<T>::resize_nocopy(newCapacity);
+        List<T>::resize_copy(count, newCapacity);
     }
     else
     {
-        List<T>::resize_copy(currLen, newCapacity);
+        List<T>::resize_nocopy(newCapacity);
     }
 
     capacity_ = List<T>::size();
@@ -84,9 +90,9 @@ inline void Foam::DynamicField<T, SizeMin>::doCapacity
 
 
 template<class T, int SizeMin>
-inline void Foam::DynamicField<T, SizeMin>::doReserve
+inline void Foam::DynamicField<T, SizeMin>::doReserve_copy
 (
-    const bool nocopy,
+    label count,
     const label len
 )
 {
@@ -95,6 +101,12 @@ inline void Foam::DynamicField<T, SizeMin>::doReserve
         // Preserve addressed size
         const label currLen = List<T>::size();
 
+        // The count truncated by the addressable range
+        if (count > currLen)
+        {
+            count = currLen;
+        }
+
         // Consistent allocated sizing
         List<T>::setAddressableSize(capacity_);
 
@@ -102,13 +114,13 @@ inline void Foam::DynamicField<T, SizeMin>::doReserve
         capacity_ =
             Foam::ListPolicy::reserve_size<SizeMin, 2>(len, capacity_);
 
-        if (nocopy)
+        if (count > 0)
         {
-            List<T>::resize_nocopy(capacity_);
+            List<T>::resize_copy(count, capacity_);
         }
         else
         {
-            List<T>::resize_copy(currLen, capacity_);
+            List<T>::resize_nocopy(capacity_);
         }
 
         capacity_ = List<T>::size();
@@ -117,18 +129,6 @@ inline void Foam::DynamicField<T, SizeMin>::doReserve
 }
 
 
-template<class T, int SizeMin>
-inline void Foam::DynamicField<T, SizeMin>::doResize
-(
-    const bool nocopy,
-    const label len
-)
-{
-    this->doReserve(nocopy, len);
-    List<T>::setAddressableSize(len);
-}
-
-
 // * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 template<class T, int SizeMin>
@@ -342,7 +342,7 @@ inline Foam::DynamicField<T, SizeMin>::DynamicField
 )
 :
     Field<T>(),
-    capacity_(content.size())
+    capacity_(0)
 {
     if (reuse)
     {
@@ -352,6 +352,7 @@ inline Foam::DynamicField<T, SizeMin>::DynamicField
     {
         Field<T>::operator=(content);
     }
+    capacity_ = Field<T>::size();
 }
 
 
@@ -424,7 +425,8 @@ inline void Foam::DynamicField<T, SizeMin>::setCapacity
     const label len
 )
 {
-    this->doCapacity(false, len);  // nocopy = false
+    // Preserve all content
+    doCapacity_copy(List<T>::size(), len);
 }
 
 
@@ -434,7 +436,8 @@ inline void Foam::DynamicField<T, SizeMin>::setCapacity_nocopy
     const label len
 )
 {
-    this->doCapacity(true, len);  // nocopy = true
+    // Preserve 0 content
+    doCapacity_copy(0, len);
 }
 
 
@@ -444,7 +447,8 @@ inline void Foam::DynamicField<T, SizeMin>::reserve
     const label len
 )
 {
-    this->doReserve(false, len);  // nocopy = false
+    // Preserve all content
+    doReserve_copy(List<T>::size(), len);
 }
 
 
@@ -454,7 +458,8 @@ inline void Foam::DynamicField<T, SizeMin>::reserve_nocopy
     const label len
 )
 {
-    this->doReserve(true, len);  // nocopy = true
+    // Preserve 0 content
+    doReserve_copy(0, len);
 }
 
 
@@ -472,9 +477,12 @@ inline void Foam::DynamicField<T, SizeMin>::reserve_exact
         // Consistent allocated sizing
         List<T>::setAddressableSize(capacity_);
 
+        // Exact length
+        capacity_ = len;
+
         // if (!nocopy)
         {
-            List<T>::resize_copy(currLen, len);
+            List<T>::resize_copy(currLen, capacity_);
         }
 
         capacity_ = List<T>::size();
@@ -489,17 +497,21 @@ inline void Foam::DynamicField<T, SizeMin>::resize
     const label len
 )
 {
-    this->doResize(false, len);  // nocopy = false
+    // Preserve all content
+    resize_copy(List<T>::size(), len);
 }
 
 
 template<class T, int SizeMin>
-inline void Foam::DynamicField<T, SizeMin>::resize_nocopy
+inline void Foam::DynamicField<T, SizeMin>::resize_copy
 (
+    label count,
     const label len
 )
 {
-    this->doResize(true, len);  // nocopy = true
+    // Preserve count elements
+    doReserve_copy(count, len);
+    List<T>::setAddressableSize(len);
 }
 
 
@@ -510,11 +522,22 @@ inline void Foam::DynamicField<T, SizeMin>::resize_fill
     const T& val
 )
 {
-    this->doResize(true, len);  // nocopy = true
+    resize_nocopy(len);
     UList<T>::operator=(val);
 }
 
 
+template<class T, int SizeMin>
+inline void Foam::DynamicField<T, SizeMin>::resize_nocopy
+(
+    const label len
+)
+{
+    // Preserve 0 content
+    resize_copy(0, len);
+}
+
+
 template<class T, int SizeMin>
 inline void Foam::DynamicField<T, SizeMin>::resize
 (
@@ -523,6 +546,8 @@ inline void Foam::DynamicField<T, SizeMin>::resize
 )
 {
     const label oldLen = List<T>::size();
+
+    // Preserve all content
     resize(len);
 
     // Fill newly exposed with constant value
@@ -546,6 +571,11 @@ inline void Foam::DynamicField<T, SizeMin>::clear() noexcept
 template<class T, int SizeMin>
 inline void Foam::DynamicField<T, SizeMin>::clearStorage()
 {
+    // Extra safety...?
+    if (!List<T>::cdata() && List<T>::empty()) capacity_ = 0;
+
+    // Consistent allocated sizing
+    List<T>::setAddressableSize(capacity_);
     List<T>::clear();
     capacity_ = 0;
 }
@@ -555,7 +585,6 @@ template<class T, int SizeMin>
 inline void Foam::DynamicField<T, SizeMin>::shrink_to_fit()
 {
     const label currLen = List<T>::size();
-
     if (currLen < capacity_)
     {
         List<T>::setAddressableSize(capacity_);
@@ -571,8 +600,11 @@ Foam::DynamicField<T, SizeMin>::swap(List<T>& other)
 {
     if
     (
-        static_cast<const List<T>*>(this)
-     == static_cast<const List<T>*>(&other)
+        FOAM_UNLIKELY
+        (
+            static_cast<const List<T>*>(this)
+         == static_cast<const List<T>*>(&other)
+        )
     )
     {
         return;  // Self-swap is a no-op
@@ -598,8 +630,11 @@ inline void Foam::DynamicField<T, SizeMin>::swap
 {
     if
     (
-        static_cast<const List<T>*>(this)
-     == static_cast<const List<T>*>(&other)
+        FOAM_UNLIKELY
+        (
+            static_cast<const List<T>*>(this)
+         == static_cast<const List<T>*>(&other)
+        )
     )
     {
         return;  // Self-swap is a no-op
@@ -622,8 +657,11 @@ inline void Foam::DynamicField<T, SizeMin>::swap
 {
     if
     (
-        static_cast<const List<T>*>(this)
-     == static_cast<const List<T>*>(&other)
+        FOAM_UNLIKELY
+        (
+            static_cast<const List<T>*>(this)
+         == static_cast<const List<T>*>(&other)
+        )
     )
     {
         return;  // Self-swap is a no-op
@@ -633,17 +671,19 @@ inline void Foam::DynamicField<T, SizeMin>::swap
     UList<T>::swap(other);
 
     // Swap capacity
-    const label oldCap = this->capacity();
-    const label newCap = other.capacity();
-
-    this->setCapacity_unsafe(newCap);
-    other.setCapacity_unsafe(oldCap);
+    auto old = capacity_;
+    this->setCapacity_unsafe(other.capacity());
+    other.setCapacity_unsafe(old);
 }
 
 
 template<class T, int SizeMin>
 inline void Foam::DynamicField<T, SizeMin>::transfer(List<T>& list)
 {
+    // No check for self-assignment (different types)
+
+    // Consistent allocated sizing
+    List<T>::setAddressableSize(capacity_);
     Field<T>::transfer(list);
     capacity_ = Field<T>::size();
 }
@@ -653,7 +693,7 @@ template<class T, int SizeMin>
 template<int AnySizeMin>
 inline void Foam::DynamicField<T, SizeMin>::transfer
 (
-    DynamicList<T, AnySizeMin>& list
+    DynamicList<T, AnySizeMin>& other
 )
 {
     if
@@ -661,19 +701,22 @@ inline void Foam::DynamicField<T, SizeMin>::transfer
         FOAM_UNLIKELY
         (
             static_cast<const UList<T>*>(this)
-         == static_cast<const UList<T>*>(&list)
+         == static_cast<const UList<T>*>(&other)
         )
     )
     {
         return;  // Self-assignment is a no-op
     }
 
-    // Consistent allocated sizing
-    List<T>::setAddressableSize(capacity_);
-    Field<T>::transfer(static_cast<List<T>&>(list));
+    // Remove storage
+    this->clearStorage();
 
-    capacity_ = list.capacity();
-    list.setCapacity_unsafe(0);  // All contents moved
+    // Swap storage and addressable size
+    UList<T>::swap(other);
+
+    // Update capacity
+    capacity_ = other.capacity();
+    other.setCapacity_unsafe(0);
 }
 
 
@@ -681,7 +724,7 @@ template<class T, int SizeMin>
 template<int AnySizeMin>
 inline void Foam::DynamicField<T, SizeMin>::transfer
 (
-    DynamicField<T, AnySizeMin>& list
+    DynamicField<T, AnySizeMin>& other
 )
 {
     if
@@ -689,19 +732,22 @@ inline void Foam::DynamicField<T, SizeMin>::transfer
         FOAM_UNLIKELY
         (
             static_cast<const UList<T>*>(this)
-         == static_cast<const UList<T>*>(&list)
+         == static_cast<const UList<T>*>(&other)
         )
     )
     {
         return;  // Self-assignment is a no-op
     }
 
-    // Consistent allocated sizing
-    List<T>::setAddressableSize(capacity_);
-    Field<T>::transfer(static_cast<List<T>&>(list));
+    // Remove storage
+    this->clearStorage();
 
-    capacity_ = list.capacity();
-    list.setCapacity_unsafe(0);  // All contents moved
+    // Swap storage and addressable size
+    UList<T>::swap(other);
+
+    // Update capacity
+    capacity_ = other.capacity();
+    other.setCapacity_unsafe(0);
 }
 
 
diff --git a/src/Pstream/dummy/Make/files b/src/Pstream/dummy/Make/files
index 6126bd6cf7..efa2cbbb60 100644
--- a/src/Pstream/dummy/Make/files
+++ b/src/Pstream/dummy/Make/files
@@ -5,6 +5,7 @@ UPstreamCommunicator.C
 UPstreamGatherScatter.C
 UPstreamReduce.C
 UPstreamRequest.C
+UPstreamFile.C
 UPstreamWindow.C
 
 UIPstreamRead.C
diff --git a/src/Pstream/dummy/UPstreamFile.C b/src/Pstream/dummy/UPstreamFile.C
new file mode 100644
index 0000000000..c2f5d57b9d
--- /dev/null
+++ b/src/Pstream/dummy/UPstreamFile.C
@@ -0,0 +1,170 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "fileName.H"
+#include "UPstreamFile.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+
+/*---------------------------------------------------------------------------*\
+                    Class UPstream::File::Impl Declaration
+\*---------------------------------------------------------------------------*/
+
+class UPstream::File::Impl {};
+
+} // End namespace Foam
+
+
+// * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
+
+bool Foam::UPstream::File::supported()
+{
+    return false;
+}
+
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+Foam::UPstream::File::File() {}
+
+
+// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
+
+Foam::UPstream::File::~File()
+{}  // Non-default in header (incomplete types)
+
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+const Foam::fileName& Foam::UPstream::File::name() const
+{
+    return fileName::null;
+}
+
+
+bool Foam::UPstream::File::is_open() const
+{
+    return false;
+}
+
+
+bool Foam::UPstream::File::close()
+{
+    return false;
+}
+
+
+// * * * * * * * * * * * * Member Functions (Reading)  * * * * * * * * * * * //
+
+#if 0
+bool Foam::UPstream::File::open_read
+(
+    const int communicator,
+    const fileName& pathname
+)
+{
+    NotImplemented;
+    return false;
+}
+#endif
+
+
+// * * * * * * * * * * * * Member Functions (Writing)  * * * * * * * * * * * //
+
+bool Foam::UPstream::File::open_write
+(
+    const int communicator,
+    const fileName& pathname,
+    IOstreamOption::atomicType
+)
+{
+    NotImplemented;
+    return false;
+}
+
+
+bool Foam::UPstream::File::write_data
+(
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    NotImplemented;
+    return false;
+}
+
+
+bool Foam::UPstream::File::write_data_at
+(
+    std::streamsize offset,
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    NotImplemented;
+    return false;
+}
+
+
+bool Foam::UPstream::File::write_data_all
+(
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    NotImplemented;
+    return false;
+}
+
+
+bool Foam::UPstream::File::write_data_at_all
+(
+    std::streamsize offset,
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    NotImplemented;
+    return false;
+}
+
+
+bool Foam::UPstream::File::set_size(std::streamsize num_bytes)
+{
+    NotImplemented;
+    return false;
+}
+
+
+// ************************************************************************* //
diff --git a/src/Pstream/mpi/Make/files b/src/Pstream/mpi/Make/files
index e859a9209f..7752acf66e 100644
--- a/src/Pstream/mpi/Make/files
+++ b/src/Pstream/mpi/Make/files
@@ -6,6 +6,7 @@ UPstreamCommunicator.C
 UPstreamGatherScatter.C
 UPstreamReduce.C
 UPstreamRequest.C
+UPstreamFile.C
 UPstreamWindow.C
 
 UIPstreamRead.C
diff --git a/src/Pstream/mpi/UPstreamFile.C b/src/Pstream/mpi/UPstreamFile.C
new file mode 100644
index 0000000000..c7ba9b4055
--- /dev/null
+++ b/src/Pstream/mpi/UPstreamFile.C
@@ -0,0 +1,687 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2025 Mark Olesen
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+\*---------------------------------------------------------------------------*/
+
+#include "fileName.H"
+#include "UPstreamFile.H"
+#include "PstreamGlobals.H"
+#include "OSspecific.H"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+// Has _c() version?
+#undef Foam_UPstream_largeCountFile
+
+#if defined(OMPI_MAJOR_VERSION)
+    #if (OMPI_MAJOR_VERSION >= 5)
+        #define Foam_UPstream_largeCountFile
+    #endif
+#endif
+
+// Macros for calling versions with or without '_c'
+#ifdef Foam_UPstream_largeCountFile
+#define Foam_mpiCall(Function)  Function##_c
+#else
+#define Foam_mpiCall(Function)  Function
+#endif
+
+
+// * * * * * * * * * * * * * * * Local Functions * * * * * * * * * * * * * * //
+
+namespace
+{
+
+inline bool checkCount(std::streamsize count, const char* what)
+{
+    #ifndef Foam_UPstream_largeCountFile
+    if (FOAM_UNLIKELY(count > std::streamsize(INT_MAX)))
+    {
+        using namespace Foam;
+        FatalErrorInFunction
+            << "Write size " << label(count)
+            << " exceeds INT_MAX bytes for '" << what << "'\n"
+            << Foam::abort(Foam::FatalError);
+        return false;
+    }
+    #endif
+
+    return true;
+}
+
+}  // End anonymous namespace
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+namespace Foam
+{
+
+/*---------------------------------------------------------------------------*\
+                    Class UPstream::File::Impl Declaration
+\*---------------------------------------------------------------------------*/
+
+class UPstream::File::Impl
+{
+    //- The file-handle
+    MPI_File fh_;
+
+    //- Path of the open file
+    fileName name_;
+
+    //- The current state (open|read|write|closed etc)
+    int state_;
+
+    //- The associated rank when openned
+    int rank_;
+
+public:
+
+    //- The file states
+    enum states : int { CLOSED = 0, READ, WRITE, ATOMIC_WRITE };
+
+    // Constructors
+
+        //- Default construct
+        Impl()
+        :
+            fh_(MPI_FILE_NULL),
+            state_(CLOSED),
+            rank_(0)
+        {}
+
+
+    // Member Functions
+
+        // The file handle
+        const MPI_File& handle() const noexcept { return fh_; }
+        MPI_File& handle() noexcept { return fh_; }
+
+        // Path to the open file
+        const fileName& name() const noexcept { return name_; }
+        fileName& name() noexcept { return name_; }
+
+        // Change the file state, return the old value
+        int state(states val) noexcept
+        {
+            int old(state_);
+            state_ = val;
+            return old;
+        }
+
+        //- Is rank 0 ? (master rank)
+        bool master() const noexcept { return (rank_ == 0); }
+
+        //- Get the associated rank
+        int rank() const noexcept { return rank_; }
+
+        //- Set the associated rank
+        void rank(int val) noexcept { rank_ = val; }
+
+
+    // Checks
+
+        // The file state
+        bool is_open() const noexcept { return state_; }
+
+        // The file read state
+        bool is_read() const noexcept
+        {
+            return (states::READ == state_);
+        }
+
+        // The file write atomic state
+        bool is_atomic() const noexcept
+        {
+            return (states::ATOMIC_WRITE == state_);
+        }
+
+        // The file write state (atomic or non-atomic)
+        bool is_write() const noexcept
+        {
+            return (states::ATOMIC_WRITE == state_ || states::WRITE == state_);
+        }
+
+        //- Assert is_read() or FatalError
+        inline bool checkReadable(const char* what) const
+        {
+            if (FOAM_UNLIKELY(!is_read()))
+            {
+                FatalErrorInFunction
+                    << "File handler not open for reading '" << what << "'\n"
+                    << "name: " << name() << nl
+                    << Foam::exit(Foam::FatalError);
+                return false;
+            }
+            return true;
+        }
+
+        //- Assert is_write() or FatalError
+        inline bool checkWritable(const char* what) const
+        {
+            if (FOAM_UNLIKELY(!is_write()))
+            {
+                FatalErrorInFunction
+                    << "File handler not open for writing'" << what << "'\n"
+                    << "name: " << name() << nl
+                    << Foam::exit(Foam::FatalError);
+                return false;
+            }
+            return true;
+        }
+};
+
+} // End namespace Foam
+
+
+// * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
+
+bool Foam::UPstream::File::supported()
+{
+    return true;
+}
+
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
+
+Foam::UPstream::File::File()
+:
+    file_(new UPstream::File::Impl)
+{}
+
+
+// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //
+
+Foam::UPstream::File::~File()
+{
+    if (FOAM_UNLIKELY(file_ && file_->is_open()))
+    {
+        WarningInFunction
+            << "Exited scope without close()" << nl
+            << "    FIX YOUR CODE!!" << endl;
+        // Do not call close() since we don't know where that collective
+        // should have been called
+    }
+}
+
+
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
+const Foam::fileName& Foam::UPstream::File::name() const
+{
+    return (file_ ? file_->name() : fileName::null);
+}
+
+
+bool Foam::UPstream::File::is_open() const
+{
+    return bool(file_ && file_->is_open());
+}
+
+
+bool Foam::UPstream::File::close()
+{
+    if (FOAM_UNLIKELY(!file_->is_open()))
+    {
+        WarningInFunction
+            << "Called without an open file handler !" << endl;
+        return false;
+    }
+
+    MPI_File_close(&(file_->handle()));
+
+    // Atomic rename of file (master only)
+    const fileName& pathname = file_->name();
+
+    if (file_->master() && file_->is_atomic() && !pathname.empty())
+    {
+        std::rename
+        (
+            (pathname + "~tmp~").c_str(),
+            pathname.c_str()
+        );
+    }
+
+    file_->state(Impl::CLOSED);
+    file_->name() = "";
+    file_->rank(0);
+
+    return true;
+}
+
+
+// * * * * * * * * * * * * Member Functions (Reading)  * * * * * * * * * * * //
+
+#if 0
+bool Foam::UPstream::File::open_read
+(
+    const int communicator,
+    const fileName& pathname
+)
+{
+    //Needed?  PstreamGlobals::checkCommunicator(communicator, 0);
+
+    if (FOAM_UNLIKELY(file_->is_open()))
+    {
+        WarningInFunction
+            << "Previous use of file handler did not call close()" << nl
+            << "    FIX YOUR CODE!!" << endl;
+        // Do not call close() since we don't know where that collective
+        // should have been called
+    }
+    file_->state(Impl::CLOSED);
+    file_->name() = pathname;  // <- set now for external error messages
+    file_->rank(0);
+
+    int returnCode = MPI_File_open
+    (
+        PstreamGlobals::MPICommunicators_[communicator],
+        pathname.c_str(),
+        (MPI_MODE_RDONLY),
+        MPI_INFO_NULL,
+        &(file_->handle())
+    );
+
+    if (FOAM_UNLIKELY(MPI_SUCCESS != returnCode))
+    {
+        FatalErrorInFunction
+            << "Error encounted in MPI_File_open() : "
+            << pathname << nl
+            << Foam::exit(Foam::FatalError);
+
+        return false;
+    }
+
+    file_->state(Impl::READ);
+    file_->name() = pathname;
+    file_->rank(UPstream::myProcNo(communicator));
+
+    return true;  // ie, is_read()
+}
+#endif
+
+
+// * * * * * * * * * * * *  Non-Collective Reading * * * * * * * * * * * * * //
+
+#if 0
+bool Foam::UPstream::File::get_header(DynamicList<char>& content)
+{
+    std::streamsize headerSize(4096);
+
+    // constexpr const char* const func = "MPI_File_read_at";
+    file_->checkReadable("MPI_File_read_at");
+
+    if (off_t fileLen = Foam::fileSize(this->name()); fileLen >= 0)
+    {
+        std::streamsize size = std::streamsize(fileLen);
+        if (headerSize > size)
+        {
+            headerSize = size;
+        }
+    }
+    else
+    {
+        content.clear();
+        return false;
+    }
+
+    // Get the first header content:
+    content.resize_nocopy(headerSize);
+
+    int returnCode = Foam_mpiCall(MPI_File_read_at)
+    (
+        file_->handle(),
+        0,  // offset
+        content.data(),
+        content.size(),
+        MPI_BYTE,
+        MPI_STATUS_IGNORE
+    );
+
+    // Wrap as ISpanStream headerStream(content);
+    if (MPI_SUCCESS == returnCode)
+    {
+        ISpanStream is(content);
+        dictionary headerDict;
+
+        // Read the regular "FoamFile" header
+        bool ok = io.readHeader(headerDict, is);
+
+        // Probably collated - extract class from "data.class"
+        if
+        (
+            decomposedBlockData::isCollatedType(io)
+         && headerDict.readIfPresent("data.class", io.headerClassName())
+        )
+        {
+            return ok;
+        }
+    }
+
+    return (MPI_SUCCESS == returnCode);
+}
+#endif
+
+
+// * * * * * * * * * * * * Member Functions (Writing)  * * * * * * * * * * * //
+
+bool Foam::UPstream::File::open_write
+(
+    const int communicator,
+    const fileName& pathname,
+    IOstreamOption::atomicType atomicType
+)
+{
+    //Needed?  PstreamGlobals::checkCommunicator(communicator, 0);
+
+    if (FOAM_UNLIKELY(file_->is_open()))
+    {
+        WarningInFunction
+            << "Previous use of file handler did not call close()" << nl
+            << "    FIX YOUR CODE!!" << endl;
+        // Do not call close() since we don't know where that collective
+        // should have been called
+    }
+    file_->state(Impl::CLOSED);
+    file_->name() = pathname;  // <- set now for external error messages
+    file_->rank(0);
+
+    const bool atomic = (IOstreamOption::atomicType::ATOMIC == atomicType);
+
+    // When opening new files, remove file variants out of the way.
+    // Eg, opening "file1"
+    // - remove old "file1.gz" (compressed)
+    // - also remove old "file1" if it is a symlink
+
+    const fileName pathname_gz(pathname + ".gz");
+    const fileName pathname_tmp(pathname + "~tmp~");
+
+    // File to open with MPI_File_open
+    const auto& target = (atomic ? pathname_tmp : pathname);
+
+    // Remove old compressed version (if any)
+    if
+    (
+        auto fType = Foam::type(pathname_gz, false);
+        (fType == fileName::SYMLINK || fType == fileName::FILE)
+    )
+    {
+        Foam::rm(pathname_gz);
+    }
+
+    // Avoid writing into symlinked files (non-append mode)
+    if
+    (
+        auto fType = Foam::type(target, false);
+        fType == fileName::SYMLINK
+    )
+    {
+        Foam::rm(target);
+    }
+
+    int returnCode = MPI_File_open
+    (
+        PstreamGlobals::MPICommunicators_[communicator],
+        target.c_str(),
+        (MPI_MODE_CREATE | MPI_MODE_WRONLY),
+        MPI_INFO_NULL,
+        &(file_->handle())
+    );
+
+    if (FOAM_UNLIKELY(MPI_SUCCESS != returnCode))
+    {
+        FatalErrorInFunction
+            << "Error encounted in MPI_File_open() : "
+            << target << nl
+            << Foam::exit(Foam::FatalError);
+
+        return false;
+    }
+
+    file_->state(atomic ? Impl::ATOMIC_WRITE : Impl::WRITE);
+    file_->name() = pathname;
+    file_->rank(UPstream::myProcNo(communicator));
+
+    return true;  // ie, is_write()
+}
+
+
+bool Foam::UPstream::File::set_size(std::streamsize num_bytes)
+{
+    if (FOAM_UNLIKELY(!file_->is_open()))
+    {
+        WarningInFunction
+            << "Called without an open file handler !" << endl;
+        return false;
+    }
+
+    int returnCode = MPI_File_set_size(file_->handle(), num_bytes);
+
+    return (MPI_SUCCESS == returnCode);
+}
+
+
+// * * * * * * * * * * * *  Non-Collective Writing * * * * * * * * * * * * * //
+
+bool Foam::UPstream::File::write_data
+(
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+
+    // constexpr const char* const func = "MPI_File_write";
+    file_->checkWritable("MPI_File_write");
+    checkCount(count, "MPI_File_write");
+
+    int returnCode = Foam_mpiCall(MPI_File_write)
+    (
+        file_->handle(),
+        data,
+        count,
+        datatype,
+        MPI_STATUS_IGNORE
+    );
+
+    return (MPI_SUCCESS == returnCode);
+}
+
+
+bool Foam::UPstream::File::write_data_at
+(
+    std::streamsize offset,
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+
+    // constexpr const char* const func = "MPI_File_write_at";
+    file_->checkWritable("MPI_File_write_at");
+    checkCount(count, "MPI_File_write_at");
+
+    int returnCode = Foam_mpiCall(MPI_File_write_at)
+    (
+        file_->handle(),
+        offset,
+        data,
+        count,
+        datatype,
+        MPI_STATUS_IGNORE
+    );
+
+    return (MPI_SUCCESS == returnCode);
+}
+
+
+// * * * * * * * * * * * * *  Collective Writing * * * * * * * * * * * * * * //
+
+bool Foam::UPstream::File::write_data_all
+(
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+
+    // constexpr const char* const func = "MPI_File_write_all";
+    file_->checkWritable("MPI_File_write_all");
+    checkCount(count, "MPI_File_write_all");
+
+    int returnCode = Foam_mpiCall(MPI_File_write_all)
+    (
+        file_->handle(),
+        data,
+        count,
+        datatype,
+        MPI_STATUS_IGNORE
+    );
+
+    return (MPI_SUCCESS == returnCode);
+}
+
+
+bool Foam::UPstream::File::write_data_at_all
+(
+    std::streamsize offset,
+    const void* data,
+    std::streamsize count,
+    const UPstream::dataTypes dataTypeId
+)
+{
+    MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+
+    // constexpr const char* const func = "MPI_File_write_at_all";
+    file_->checkWritable("MPI_File_write_at_all");
+    checkCount(count, "MPI_File_write_at_all");
+
+    int returnCode = Foam_mpiCall(MPI_File_write_at_all)
+    (
+        file_->handle(),
+        offset,
+        data,
+        count,
+        datatype,
+        MPI_STATUS_IGNORE
+    );
+
+    return (MPI_SUCCESS == returnCode);
+}
+
+
+// bool Foam::UPstream::File::write_data_all_begin
+// (
+//     const void* data,
+//     std::streamsize count,
+//     const UPstream::dataTypes dataTypeId
+// )
+// {
+//     MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+//
+//     // constexpr const char* const func = "MPI_File_write_all_begin";
+//     file_->checkWritable("MPI_File_write_all_begin");
+//     checkCount(count, "MPI_File_write_all_begin");
+//
+//     int returnCode = Foam_mpiCall(MPI_File_write_all_begin)
+//     (
+//         file_->handle(),
+//         data,
+//         count,
+//         datatype,
+//         MPI_STATUS_IGNORE
+//     );
+//
+//     return (MPI_SUCCESS == returnCode);
+// }
+
+
+// bool Foam::UPstream::File::write_data_all_end
+// (
+//     const void* data
+// )
+// {
+//     file_->checkWritable("MPI_File_write_all_end");
+//     int returnCode = Foam_mpiCall(MPI_File_write_all_end)
+//     (
+//         file_->handle(),
+//         data
+//         MPI_STATUS_IGNORE
+//     );
+//
+//     return (MPI_SUCCESS == returnCode);
+// }
+
+
+// bool Foam::UPstream::File::write_data_at_all_begin
+// (
+//     std::streamsize offset,
+//     const void* data,
+//     std::streamsize count,
+//     const UPstream::dataTypes dataTypeId
+// )
+// {
+//     MPI_Datatype datatype = PstreamGlobals::getDataType(dataTypeId);
+//
+//     // constexpr const char* const func = "MPI_File_write_at_all_begin";
+//     file_->checkWritable("MPI_File_write_at_all_begin");
+//     checkCount(count, "MPI_File_write_at_all_begin");
+//
+//     int returnCode = Foam_mpiCall(MPI_File_write_at_all_begin)
+//     (
+//         file_->handle(),
+//         offset,
+//         data,
+//         count,
+//         datatype,
+//         MPI_STATUS_IGNORE
+//     );
+//
+//     return (MPI_SUCCESS == returnCode);
+// }
+
+
+// bool Foam::UPstream::File::write_data_at_all_end
+// (
+//     const void* data
+// )
+// {
+//     file_->checkWritable("MPI_File_write_at_all_end");
+//
+//     int returnCode = Foam_mpiCall(MPI_File_write_at_all_end)
+//     (
+//         file_->handle(),
+//         data
+//         MPI_STATUS_IGNORE
+//     );
+//
+//     return (MPI_SUCCESS == returnCode);
+// }
+
+
+// ************************************************************************* //