Copy-on-Write Performance and Debugging

I had to read an early blog to figure out what it was:

https://devblogs.microsoft.com/engineering-at-microsoft/dev-...

"Copy-on-write (CoW) linking, also known as block cloning in the Windows API documentation, avoids fully copying a file by creating a metadata reference to the original data on-disk. CoW links are like hardlinks but are safe to write to, as the filesystem lazily copies the original data into the link as needed when opened for append or random-access write. With a CoW link you save disk space and time since the link consists of a small amount of metadata and they write fast."

It seems there is a MacOS implementation: https://github.com/dotnet/runtime/pull/79243

But it seems that this is .Net specific and not something that would speed up other build systems? It is confusing if this can apply to other build technologies other than .NET. Can it speed up TypeScript/JavaScript builds? Can it speed up Rust builds? Also what are the speed ups on these other platforms like MacOS and Linux?

Is this something that all build systems and all OSes would benefit from?

I guess this blog post for me raises more questions than it answers.

Have been running ReFS on a drive on my Windows 10 workstation for about three years, and recently started using a dev drive equivalent on Windows 10 for the past two months. Our Unreal Engine project is quite large, 600+GB straight from the P4 depot before building. I need to keep a few separate workspaces around, one for current development work, one for swarm reviews, one for "let me test out a thing that might break" because as we know, branching in Perforce can quite painful, especially on large depots. At one point I needed to have dozens of workspaces synced to specific changelists whilst we hunted down a bug in one of our levels.

ReFS, with block de-duplication and LZ4 compression has reduced the per-workspace footprint to around 10% of what it was previously. Decreased build times by around 5% and decreased archive, stage and package times by about 80% by deploying MSBuild SDK CopyOnWrite. I also moved the DDC onto the VHDX where the project resides which has further reduced the footprint of the project.

Windows 11 canary channel (still in canary I think) has a modified Win32 that supports CoW FileCopyEx. You can get similar gains by other means on Win10 and Win11 by using ReFS CoW aware utilities.

Have used XFS, BTRFS, APFS and others extensively over the years, so I am glad that Windows is finally getting in on the action.

> We ran into this problem on one machine that had run continuous CoW builds for weeks under a prerelease CoW-in-Win32 implementation, so we don’t expect this to appear in the wild very often.

That's not exactly confidence inspiring.

You know, I've always been kinda amused that something very simple like "cat a b >c" or even "fa = open("a", O_APPEND | O_WRONLY); fb = open("b", O_RDONLY); sendfile(fa, fb, NULL, 0x7ffff000);" doesn't really have either user-visible specialized API nor under-the-hood speed ups in the FS implementations. It's just gluing two files together, it's got to be a very popular operation, about as popular as "prepend the contents of file A to file B". But you can't do it in-place which is kinda annoying when you have to preserve the existing files' attributes.

I think they use a lot of extra words to say that ReFS will support the equivalent of cp --reflink.

Oh man. My dream Git Successor combines a Virtual File System with a Copy-on-Write cache to allow repos to trivially commit all their dependencies including compiler toolchains.

Windows having CoW makes my far fetched dream a possibility.

Does anyone know if there is a way to convert the entire windows installation and all files into a Dev Drive format? Without losing any data or corruption

>Dev Drive was released

I tried that Dev Drive thing and I havent seen perf improvement when building C++ code, sadly.

And with all that said WSL2 still buffers file transfers in RAM...

Crazy to see Microsoft talking about performance like they have any expertise in the matter.