On Wed, 1 Dec 2010 21:46:03 +0100
Goffredo Baroncelli <kreijack@xxxxxxxxx> wrote:
> On Wednesday, 01 December, 2010, Jeff Layton wrote:
> > A more common use case than CIFS or samba is going to be things like
> > backup programs. They commonly look at inode numbers in order to
> > identify hardlinks and may be horribly confused when there files that
> > have a link count >1 and inode number collisions with other files.
> >
> > That probably qualifies as an "enterprise-ready" show stopper...
>
> I hope that a backup program, uses the pair (inode,fsid) to identify if two
> file are hardlinked... otherwise a backup of two filesystem mounted can be
> quite danguerous...
>
>
> From the statfs(2) man page:
> [..]
> The f_fsid field
> [...]
> 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
> file-system type. Several OSes 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 file system when NFS-exported, and giving it out
> is a security concern.
>
>
> And the btrfs_statfs function returns a different fsid for every subvolume.
>
Ahh, interesting. I've never read that blurb on f_fsid...
Unfortunately, it looks like not all filesystems fill that field out.
NFS and CIFS leave it conspicuously blank. Those are probably bugs...
OTOH, the GLibc docs say this:
dev_t st_dev
Identifies the device containing the file. The st_ino and st_dev,
taken together, uniquely identify the file. The st_dev value is not
necessarily consistent across reboots or system crashes, however.
...and it's always been my understanding that a st_dev/st_ino
combination should be unique.
Is there some definitive POSIX statement on why one should prefer to
use f_fsid over st_dev in this situation?
--
Jeff Layton <jlayton@xxxxxxxxxx>
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