Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain <anajain.sg@gmail.com>]

> So if the two file systems are identical, the (f_fsid, ino) will
> uniquely determines a file.  And that's *fine* if f_fsid is the same
> for statfs(A) and statfs(B).


Got it. Do you mean that since both filesystems are identical,
statfs(A) and statfs(B) can legitimately return the same values?

And if users want them to diverge, the expectation is that they
should update the UUID (via tune2fs), after which f_fsid would
also differ. If the UUID isn't changed, then the ambiguity is
effectively a user responsibility.

I'm not entirely sure what the correct expectation for f_fsid
should be.

My initial idea was to make f_fsid behavior consistent across
major filesystems so that user space benefits from predictable
semantics.

Currently, XFS and Btrfs provide f_fsid values that are unique
per filesystem, including clones, while ext4 provides uniqueness
only if the UUID is changed by the user.

That said, it may be fine for ext4 to retain this behavior,
since statfs(2) itself has some inherent ambiguity.

If there are no further concerns, I'll update the submitted
test case generic/791 accordingly.

Thanks, Anand