Btrfs devel - Sep 2009 - Discrepant st_rdev and st_dev stats in btrfs block and fs device ids

Hello,

I think I found a btrfs 0.19 problem when used in an lvm2 volume;
btrfs is returning a st_dev stat from the mounted filesystem / that is
different from the lvm2 blockdevice st_rdev stat in
/dev/mapper/vg0-os0.

This btrfs behavior is different from ext4, where the st_dev and
st_rdev stats match (http://pastebin.com/m64e90a8e or a summary
below). Because of that, in grub2, ''grub-probe -t device / ''
fails
with '' grub-probe: error: cannot find a device for /. '' , and
so
update-grub2 fails causing cascading problems all around.

Can anyone repeat this? Any ideas why this could be happening? Is this expected?

Thanks,
Marcus
--
# uname -a
Linux frog 2.6.31-9-generic #29-Ubuntu SMP Sun Aug 30 17:39:23 UTC
2009 i686 GNU/Linux

# btrfs-show
Label: none  uuid: c5bffbe1-6968-4eee-a800-8346df56459a
    Total devices 1 FS bytes used 2.15GB
    devid    1 size 10.00GB used 10.00GB path /dev/mapper/vg0-os0
Btrfs Btrfs v0.19

# mount
/dev/mapper/vg0-os0 on / type btrfs (rw,errors=remount-ro)
/dev/mapper/vg0-os0 on /tmp/os0 type btrfs (rw)
/dev/mapper/vg0-os1 on /tmp/os1 type ext4 (rw)


stat.st_rdev for /dev/mapper/vg0-os0:64512
stat.st_rdev for /dev/mapper/vg0-os1:64513
stat.st_dev for /:16
stat.st_dev for /tmp/os0:16
stat.st_dev for /tmp/os1:64513

--
PS:
Btw, in case someone else comes across this problem, a quick
workaround for grub2 is to modify /usr/sbin/grub-mkconfig so that
GRUB_DEVICE points directly to your btrfs lvm2 block device:

# diff grub-mkconfig.orig grub-mkconfig
1122c122
< GRUB_DEVICE="`${grub_probe} --target=device /`"
---
> GRUB_DEVICE="/dev/mapper/vg0-os0"
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On Sun, Sep 06, 2009 at 04:51:55PM +0100, Marcus wrote:
> Hello,
> 
> I think I found a btrfs 0.19 problem when used in an lvm2 volume;
> btrfs is returning a st_dev stat from the mounted filesystem / that is
> different from the lvm2 blockdevice st_rdev stat in
> /dev/mapper/vg0-os0.
> 
> This btrfs behavior is different from ext4, where the st_dev and
> st_rdev stats match (http://pastebin.com/m64e90a8e or a summary
> below). Because of that, in grub2, ''grub-probe -t device /
'' fails
> with '' grub-probe: error: cannot find a device for /. '' ,
and so
> update-grub2 fails causing cascading problems all around.
> 
> Can anyone repeat this? Any ideas why this could be happening? Is this
expected?
>
Yes this is expected.  Because of the way that btrfs can have multiple disks and
snapshot and such, it creates this sort of virtual device as its backing device
in the kernel to keep track of everything, so it''s st_dev will not
match the
actual device it resides on.  Thanks,

Josef 
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Hey,
> Can anyone repeat this? Any ideas why this could be happening?
> Is this expected?
There was some discussion about this some time ago, see

 http://news.gmane.org/find-root.php?message_id=%3c7b13a0f80904120735r41736455hb7e226ab8ad30bd7%40mail.gmail.com%3e

I don''t think it''s resolved yet - the work-around I currently
use in
devkit-disks-daemon (which is used by e.g. GNOME to show names
and icons for storage devices) looks like this:

 http://cgit.freedesktop.org/DeviceKit/DeviceKit-disks/tree/src/devkit-disks-mount-monitor.c?id=006#n351

and I don''t think this will work for multi-disk btrfs filesystems.
Thinking
about this again, we probably want to treat such btrfs multi-disk filesystems
in the UI the same way we treat e.g. md raid devices....

    David
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