Simplify copy-reflink code

[Ralph Boehme]

Hi David,

thanks again for taking a look!

On 4/5/24 07:21, David Disseldorp wrote:
> I don't think dup-extents should fallback to copy; with the initial
> implementation we had VFS_COPY_CHUNK_FL_MUST_CLONE to make this
> explicit. However, the MS-FSCC spec doesn't appear to state that cloning
> is a hard requirement, only that it should be supported alongside
> FILE_SUPPORTS_BLOCK_REFCOUNTING and that offsets+lengths need to be
> "logical cluster boundary" aligned.

MS-FSA 2.1.5.9.4 FSCTL_DUPLICATE_EXTENTS_TO_FILE makes it pretty clear 
what should be done and that a fallback to data copy is not in scope.

> We probably need to do some testing against modern ReFS to check some of
> these questions.


I'm working with a customer who is testing with a Windows server with 
ReFS where he uses a testing program [1] running on the client that 
allows manual control over the operation and results confirm the spec: 
no fallback, failure on non aligned IO request.

[1] https://github.com/tdewin/refs-fclone

Thanks!
-slow
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