F15: ugly behavior of "df"

[Karel Zak]

On Fri, Jun 24, 2011 at 09:43:32AM +0200, Andreas Schwab wrote:
> Karel Zak <kzak at redhat.com> writes:
> 
> > The 'bind' flag is another way how to achieve that the filesystem is
> > mounted on another place. Nothing other.

 Pedantic note, there are some extra features usable with MS_BIND,
 like MS_UNBINDABLE, MS_REC, etc.

> >    # mount /dev/sdb1 /mnt/A
> >    # mount --bind /mnt/A /mnt/B
> >
> > is the same thing as:
> >
> >    # mount /dev/sdb1 /mnt/A
> >    # mount /dev/sdb1 /mnt/B 
> 
> There is a big difference between "mount --bind /mnt/A/B /mnt/B" and
> "mount /dev/sdb1 /mnt/B".

 Sure. The difference is that the mountpoint is not connected with root
 of the filesystem, but with some subdirectory. This information is
 stored in /proc/self/mountinfo.
 
 The important is that there is not dependence on the original
 mountpoint -- this fact was ignored in mtab, because mtab info about
 bind mounts was always based on the paths (e.g. /mnt/A/B) instead on
 devices (e.g. /dev/sdb1).
 
 BTW, this is not specific to bind mounts, you can do the same with
 btrfs subvolumes. 

  # mount /dev/sdb1 /mnt/A
  # btrfs subvolume create /mnt/A/B
  # mount /dev/sdb1 /mnt/B subvol=B

  $ df | grep sdb1
  /dev/sdb1              1022976        64    892032   1% /mnt/A
  /dev/sdb1              1022976        64    892032   1% /mnt/B

  $ findmnt /dev/sdb1
  TARGET SOURCE        FSTYPE OPTIONS
  /mnt/A /dev/sdb1     btrfs  rw,relatime,ssd
  /mnt/B /dev/sdb1[/B] btrfs  rw,relatime,ssd


 Anyway, let's fix the utils to be usable with Linux VFS independently
 on mtab.

    Karel

-- 
 Karel Zak  <kzak at redhat.com>
 http://karelzak.blogspot.com