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

["Theodore Tso" <tytso@mit.edu>]

On Thu, Apr 09, 2026 at 06:28:32AM +0800, Anand Jain wrote:
> Some A/B testing use cases require the filesystem to remain
> byte-for-byte identical. In those scenarios, changing the UUID
> isn't an option.

But in that case, where the file systems A and B are bit-fot-bit
identical, why do we care whether statfs returns different fsid's for
statfs(A) and statfs(B).  After all, f_fsid is only defined as "File
system ID", and there is no other definition.  Which is why I say that
people *really* shouldn't depending on its semantics, because it's not
well defined.

Quoting from the statfs(2) man page:

    Nobody knows what f_fsid is supposed to contain (but see below).

    ...

    The general idea is that f_fsid contains some random stuff such
    that the pair (f_fsid,ino) uniquely determines a file. Some
    operating systems use (a variation on) the device number, or the
    device number combined with the filesystem type. Several operating
    systems restrict giving out the f_fsid field to the superuser only
    (and zero it for unprivileged users), because this field is used
    in the filehandle of the filesystem when NFS-exported, and giving
    it out is a security concern.

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).

No?

					- Ted