[PATCH v3 0/5] Implement reflink support

[Alex Xu (Hello71)]

Excerpts from Fabian Maurer's message of January 27, 2022 5:24 pm:
> Hello,
> 
> first, thanks for working on this, would be a welcome improvement!
> Do I understand that right that it wouldn't work for ext4 and there's not
> really anything we could do to support it?
> 
> Regards,
> Fabian Maurer

ext4 does not support reflink, so copy_file_range cannot reduce disk 
space usage. However, it will still allow the kernel to do the file copy 
itself, avoiding copying the data to userspace.

Full reflink functionality with arbitrary offsets and overwrite is 
non-trivial to implement in the filesystem. However, we really only need 
the limited functionality of a whole-file reflink. This could be 
simulated using a hard or symbolic link which gets broken by Wine when 
any write occurs. However, as with reparse points, this requires 
modifying every file access that Wine makes, which would be a 
significant undertaking. Additionally, as explained in the cover letter, 
this would potentially break Unix-side programs modifying the Wine 
prefix, such as winetricks.

Alex.