The Microsoft.Build.CopyOnWrite MSBuild SDK overrides the native MSBuild Copy task to add support for ReFS and Dev Drive CloneFile (Copy on Write or CoW) on Windows. It is designed to be as backwards compatible as possible and should directly replace all usages of Copy in MSBuild.
On Linux and Mac the current behavior is to always fall back to regular file copies (File.Copy), however File.Copy automatically uses CoW for Linux (starting in .NET 7) and Mac (.NET 8). A similar PR for Windows did not make it into .NET, however there is work underway to integrate CoW into the Windows API in a possible future release.
This is intended to be used in a large repo already onboarded to Central Package Management. In your Directory.Packages.props:
<Project>
<ItemGroup>
<!-- <PackageVersion> elements here -->
</ItemGroup>
<ItemGroup>
<GlobalPackageReference Include="Microsoft.Build.CopyOnWrite" Version="1.0.0" />
</ItemGroup>
</Project>This example will include the Microsoft.Build.CopyOnWrite task for all NuGet-based projects in your repo.
If your project types don't support NuGet (e.g. .vcxproj), you can alternatively import this as an MSBuild SDK. In your Directory.Build.targets file:
<Project>
<Sdk Name="Microsoft.Build.CopyOnWrite" Version="1.0.0" />
<!-- ... -->
</Project>This SDK aims to improve the performance of large repositories by minimizing file copies during the build process. Many large repos have the snowball effect where Resolve Assembly Reference will add more and more dependencies that get copied along in different layers of the build graph. This is an attempt to accelerate those by taking advantage of the Copy on Write feature of the OS using this library: https://github.com/microsoft/CopyOnWrite.
Example build, internal Microsoft repo with 758 nodes, 3989 edges, on Win11 22H2 ReFS, with the NuGet cache moved onto the same disk volume:
| Command | Value |
|---|---|
msbuild /p:DisableCopyOnWrite=true |
Time Elapsed 00:06:02.03 |
| Enlistment size | 66.4 GB (71,305,402,686 bytes) |
| Size on disk | 66.9 GB (71,848,480,768 bytes) |
| Command | Value |
|---|---|
msbuild /p:DisableCopyOnWrite=false |
Time Elapsed 00:04:09.32 |
| Enlistment size | 66.4 GB (71,305,402,686 bytes) |
| Size on disk | 3.83 GB (3,055,628,028 bytes) |
See blog post 1 and blog post 2 for more information on Dev Drive, copy-on-write, and moving your package caches.
To use this feature, you need run on a drive formatted with Dev Drive or ReFS on Windows. ReFS is available on Windows Server, or on Windows 11 22H2 Enterprise and Pro SKUs. Dev Drive is available on all Windows 11 SKUs and is slated for a future Windows Server release.