support file block copy on write on Windows

[danmoseley]

On ReFS, Windows supports a block copy operation, like copy on write but at a sub-file level.

https://learn.microsoft.com/en-us/windows/win32/fileio/block-cloning
https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ni-winioctl-fsctl_duplicate_extents_to_file

This has existed for several years, but is suddenly more interesting now because of the announcement of dev drive.

We should enlighten File.Copy to use this on Windows when possible, so that MSBuild, Nuget etc work extra fast on dev drive.

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Issue Details

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On ReFS, Windows supports a block copy operation, like copy on write but at a sub-file level.

https://learn.microsoft.com/en-us/windows/win32/fileio/block-cloning
https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ni-winioctl-fsctl_duplicate_extents_to_file

This has existed for several years, but is suddenly more interesting now because of the announcement of dev drive.

We should enlighten File.Copy to use this on Windows when possible, so that MSBuild, Nuget etc work extra fast on dev drive.

Author:	danmoseley
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[danmoseley]

@hamarb123 if you comment on this issue, I'll be able to assign it to you.

[hamarb123]

Comment :)

[danmoseley]

cc @rainersigwald this should speed up MSBuild if it isn't already doing hardlinks.
cc @aortiz-msft this should help Nuget, but only where it's doing File.Copy or equivalent -- it looks like you do FileStream copy in some places which wouldn't benefit .. we might want to take a look at that at some point to see whether it's necessary.

[rainersigwald]

MSBuild has an option to do hardlinks (the Copy task's UseHardlinksIfPossible parameter, exposed in common targets as $(CreateHardLinksForCopyFilesToOutputDirectoryIfPossible)) but you have to be very careful with them since modifying (what looks like) a file in your output can silently corrupt your NuGet cache or other files, so they're not widely used.

[danmoseley]

sharing today's post for context -- https://devblogs.microsoft.com/engineering-at-microsoft/dev-drive-and-copy-on-write-for-developer-performance/

[jkoritzinsky]

There's a Copy On Write MSBuild SDK that utilizes the ReFS CloneFile API for a custom implementation of the Copy MSBuild task. Might be useful.

[danmoseley]

Yep, I think @hamarb123 is going to crib from that (https://github.com/microsoft/CopyOnWrite)