On Wed, 7 Jul 2010 12:11:50 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Wed, Jul 07, 2010 at 09:36:29AM +1000, Neil Brown wrote:
> > On Wed, 7 Jul 2010 09:23:51 +1000
> > Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >
> > > > I can add a new syscall that returns
> > > >
> > > > struct fs_uuid {
> > > > u8 fs_uuid[16];
> > > > };
> > > >
> > > > long sys_get_fs_uuid(int dfd, char *name, struct fs_uuid *fsid, int flag);
> > >
> > > libblkid already provides the UUID to userspace applications, doesn't it?
> >
> > Yes and no.
> >
> > libblkid provides the uuid of the thing that uses a block device. That
> > doesn't directly map to "UUID of a filesystem".
>
> True.
>
> > There are two types of filesystem that I can think of for which libblkid
> > cannot give a uuid.
> > - network filesystems (or virtual filesystems, or fuse )
>
> How would you guarantee persistent uniqueness for such filesystems?
Persistent shouldn't be too hard in many cases.
What uniqueness guarantees do we have anyway? Mostly stochastic I expect.
>
> > - filesystems which share a block device, such as btrfs.
> > btrfs can have 'subvols' - multiple "filesystems" within
> > the one (set of) block device(s). libblkid cannot be asked about these
> > different subvols.
> >
> > libblkid is useful, but not a real solution.
>
> So libblkid doesn't cover everything, but I think my question is
> still valid - if we want per-filesystem UUIDs, why a syscall and not
> just publishing it somewhere where we already publish per-mount
> information? e.g. in /proc/mounts?
The trouble with /proc/mounts is that it is somewhat clumsy to parse
(remember to handle \0ctal escapes) and doesn't include major/minor number
which is the primary key for identifying filesystems in Linux
(see /sys/class/bdi/MAJOR:MINOR which is e.g. the best place to configure
read-ahead for a filesystem).
So /proc/mounts could work (and would probably be better than a new syscall)
but I would really rather see something sane in /sys for
inspecting/configuring filesystems (rather than each filesystem doing their
own independent thing in /sys/fs).
NeilBrown
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