Btrfs devel - Jan 2012 - fstab mount options ignored on subsequent subvolume mounts

Greeting all,

I have multiple subvolumes on the same filesystem that are mounted with
different options in fstab.
The problem is the mount options for subsequent subvolume mounts seem to be
ignored as reflected in /proc/mounts.

$ cat /etc/fstab | grep mnt
UUID=<REMOVED> /mnt/a btrfs
subvol=a,defaults,nodatacow,autodefrag,noatime,space_cache,inode_cache 0 0
UUID=<REMOVED> /mnt/b btrfs
subvol=b,defaults,autodefrag,noatime,space_cache,inode_cache 0 0
UUID=<REMOVED> /mnt/c btrfs
subvol=c,defaults,compress=zlib,autodefrag,noatime,space_cache,inode_cache 0 0
$ mount | grep mnt
/dev/sdb2 on /mnt/a type btrfs
(rw,noatime,subvol=a,nodatacow,autodefrag,space_cache,inode_cache)
/dev/sdb2 on /mnt/b type btrfs
(rw,noatime,subvol=b,autodefrag,space_cache,inode_cache)
/dev/sdb2 on /mnt/c type btrfs
(rw,noatime,subvol=c,compress=zlib,autodefrag,space_cache,inode_cache)
$ cat /proc/mounts | grep mnt
/dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
/dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
/dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0

continuing the example which should only change the mount options for one of the
subvolumes:
$ sudo mount -o remount,compress=zlib /mnt/oldhome
$ cat /proc/mounts | grep mnt
/dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
/dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
/dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0


Running Ubuntu mainline kernel 3.2.1 (3.2.1-030201-generic #201201121644 SMP Thu
Jan 12 21:53:24 UTC 2012 i686 athlon i386 GNU/Linux) with most recent
btrfs-progs (2011-12-01) from linux/kernel/git/mason/btrfs-progs.git

Thanks,
Kyle
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On Tue, Jan 17, 2012 at 01:09:36PM -0600, Kyle Gates
wrote:
> I have multiple subvolumes on the same filesystem that are mounted with
different options in fstab.
> The problem is the mount options for subsequent subvolume mounts seem to be
ignored as reflected in /proc/mounts.
The output of ''mount'' and /proc/mounts is different. mount
takes it from
/etc/mtab while /proc/mounts gets the information from kernel (calls
into super.c:btrfs_show_options() )

''mtab'':
- contains the options in order in which they were given to mount or in
  /etc/fstab
- 

/proc/mounts:
- order of options is fixed (as defined in the function)
- if the option has a default value which was not given to mount, it is
  listed here (and is not in mtab)
- an implied options appear here as well (like nodatacow implies
  nodatasum)


Now, you''re giving different set of options to each subvolume, but they
belong to one filesystem and thus will result in set of options given to
the first mounted subvolume for every other mounted subvolume.

The first subvol calls ''btrfs_fill_super'' and
''btrfs_parse_options'', the
other do not and do not. Remount will call
''btrfs_parse_options'' again
and will change the options set.
> $ cat /etc/fstab | grep mnt
> UUID=<REMOVED> /mnt/a btrfs
subvol=a,defaults,nodatacow,autodefrag,noatime,space_cache,inode_cache 0 0
> UUID=<REMOVED> /mnt/b btrfs
subvol=b,defaults,autodefrag,noatime,space_cache,inode_cache 0 0
> UUID=<REMOVED> /mnt/c btrfs
subvol=c,defaults,compress=zlib,autodefrag,noatime,space_cache,inode_cache 0 0
>
> $ mount | grep mnt
> /dev/sdb2 on /mnt/a type btrfs
(rw,noatime,subvol=a,nodatacow,autodefrag,space_cache,inode_cache)
> /dev/sdb2 on /mnt/b type btrfs
(rw,noatime,subvol=b,autodefrag,space_cache,inode_cache)
> /dev/sdb2 on /mnt/c type btrfs
(rw,noatime,subvol=c,compress=zlib,autodefrag,space_cache,inode_cache)
> $ cat /proc/mounts | grep mnt
> /dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> /dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> /dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> 
> continuing the example which should only change the mount options for one
of the subvolumes:
> $ sudo mount -o remount,compress=zlib /mnt/oldhome
> $ cat /proc/mounts | grep mnt
> /dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
> /dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
> /dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
I think the above explains things in general in your listings, the last
one missing is subvol= in /proc/mounts. This is not implemented, but is
possible (save non-default subvol name with the subvol root and print in
show_options).


david
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> > I have multiple subvolumes on the same filesystem that are mounted
with different options in fstab.
> > The problem is the mount options for subsequent subvolume mounts seem
to be ignored as reflected in /proc/mounts.
>
> The output of ''mount'' and /proc/mounts is different.
mount takes it from
> /etc/mtab while /proc/mounts gets the information from kernel (calls
> into super.c:btrfs_show_options() )
>
> ''mtab'':
> - contains the options in order in which they were given to mount or in
> /etc/fstab
> -
>
> /proc/mounts:
> - order of options is fixed (as defined in the function)
> - if the option has a default value which was not given to mount, it is
> listed here (and is not in mtab)
> - an implied options appear here as well (like nodatacow implies
> nodatasum)
>
>
> Now, you''re giving different set of options to each subvolume, but
they
> belong to one filesystem and thus will result in set of options given to
> the first mounted subvolume for every other mounted subvolume.
>
> The first subvol calls ''btrfs_fill_super'' and
''btrfs_parse_options'', the
> other do not and do not. Remount will call
''btrfs_parse_options'' again
> and will change the options set.
>
> > $ cat /etc/fstab | grep mnt
> > UUID=<REMOVED> /mnt/a btrfs
subvol=a,defaults,nodatacow,autodefrag,noatime,space_cache,inode_cache 0 0
> > UUID=<REMOVED> /mnt/b btrfs
subvol=b,defaults,autodefrag,noatime,space_cache,inode_cache 0 0
> > UUID=<REMOVED> /mnt/c btrfs
subvol=c,defaults,compress=zlib,autodefrag,noatime,space_cache,inode_cache 0 0
> >
> > $ mount | grep mnt
> > /dev/sdb2 on /mnt/a type btrfs
(rw,noatime,subvol=a,nodatacow,autodefrag,space_cache,inode_cache)
> > /dev/sdb2 on /mnt/b type btrfs
(rw,noatime,subvol=b,autodefrag,space_cache,inode_cache)
> > /dev/sdb2 on /mnt/c type btrfs
(rw,noatime,subvol=c,compress=zlib,autodefrag,space_cache,inode_cache)
> > $ cat /proc/mounts | grep mnt
> > /dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> > /dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> > /dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,space_cache,autodefrag,inode_cache 0 0
> >
> > continuing the example which should only change the mount options for
one of the subvolumes:
> > $ sudo mount -o remount,compress=zlib /mnt/oldhome
> > $ cat /proc/mounts | grep mnt
> > /dev/sdb2 /mnt/a btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
> > /dev/sdb2 /mnt/b btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
> > /dev/sdb2 /mnt/c btrfs
rw,noatime,nodatasum,nodatacow,compress=zlib,space_cache,autodefrag,inode_cache
0 0
>
> I think the above explains things in general in your listings, the last
> one missing is subvol= in /proc/mounts. This is not implemented, but is
> possible (save non-default subvol name with the subvol root and print in
> show_options).
>
>
> david
Thanks for the clarification. I was under the impression that mounting 
multiple subvolumes with different options had been implemented. Perhaps
 someday it will be although for now there are more pressing issues.


I appreciate everyone''s hard work and look forward to the continued
development of btrfs.

many thanks,
Kyle
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