BTRFS thinks that a device is mounted

BTRFS thinks that a device is mounted

[Nikos Voutsinas]

Hello all,

What went wrong in the following scenario? BTRFS thinks that a device is
mounted while it is not.

BTRFS tools ver: Btrfs v0.19-101-g3891d2d
Uname: 3.1.0-rc7-686-pae


## Comment: Initially we had only root (/) mounted as btrfs
mount |grep btrfs
/dev/sdc2 on / type btrfs (rw,autodefrag)

## Comment: Create a new btrfs fs on /dev/sdg /dev/sdh /dev/sdl
mkfs.btrfs -m raid1 -d raid1 /dev/sdg /dev/sdh /dev/sdl

btrfs filesystem show
Label: none  uuid: f0425f63-e486-4c10-adab-47a23085c8db
        Total devices 3 FS bytes used 28.00KB
        devid    3 size 2.00GB used 200.00MB path /dev/sdl
        devid    2 size 1.00GB used 200.00MB path /dev/sdh
        devid    1 size 768.00MB used 404.00MB path /dev/sdg

Label: none  uuid: cc8211ba-6f62-46d6-ba17-49250f06a941
        Total devices 1 FS bytes used 512.98MB
        devid    1 size 6.09GB used 1.75GB path /dev/sdc2

## Comment: unorthodox, but looks possible, add one of the devices from
the newly created fs to root (/)
btrfs device add /dev/sdh /

## Comment: Remove it back.
btrfs device delete /dev/sdh /


## Comment: We ended up with "*** Some devices missing" on both devices
btrfs filesystem show
Label: none  uuid: 5910f378-0784-4e7f-b918-de35067252e9
        Total devices 3 FS bytes used 28.00KB
        devid    3 size 2.00GB used 200.00MB path /dev/sdl
        devid    1 size 768.00MB used 404.00MB path /dev/sdg
        *** Some devices missing

Label: none  uuid: cc8211ba-6f62-46d6-ba17-49250f06a941
        Total devices 2 FS bytes used 512.59MB
        devid    1 size 6.09GB used 1.50GB path /dev/sdc2
        *** Some devices missing

## Comment: Use btrfs balance to fix root (/)
btrfs filesystem balance /

btrfs filesystem show
Label: none  uuid: 5910f378-0784-4e7f-b918-de35067252e9
        Total devices 3 FS bytes used 28.00KB
        devid    3 size 2.00GB used 200.00MB path /dev/sdl
        devid    1 size 768.00MB used 404.00MB path /dev/sdg
        *** Some devices missing

Label: none  uuid: cc8211ba-6f62-46d6-ba17-49250f06a941
        Total devices 1 FS bytes used 513.40MB
        devid    1 size 6.09GB used 1.75GB path /dev/sdc2

## Comment: try to mount the initially created fs on /mnt. It fails
mount -t btrfs /dev/sdg /mnt
mount: wrong fs type, bad option, bad superblock on /dev/sdg,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

## Comment: See if device scan can do the trick. Nope...
btrfs device scan /dev/sdh
Scanning for Btrfs filesystems in '/dev/sdh'
ERROR: unable to scan the device '/dev/sdh' - Invalid argument

## Comment: OK, we give up. Try to recreate btrfs from scratch
mkfs.btrfs -m raid1 -d raid1 /dev/sdg /dev/sdh /dev/sdl

WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

/dev/sdh is mounted

## Comment: Of course /dev/sdh is not mounted.
mount |grep /dev/sdh
root@lxc:~#


Thanks,
Nikos

Re: BTRFS thinks that a device is mounted

[Helmut Hullen]

Hallo, Nikos,

Du meintest am 21.10.11:

> What went wrong in the following scenario? BTRFS thinks that a device
> is mounted while it is not.

[...]

> btrfs device scan /dev/sdh
> Scanning for Btrfs filesystems in '/dev/sdh'
> ERROR: unable to scan the device '/dev/sdh' - Invalid argument

> ## Comment: OK, we give up. Try to recreate btrfs from scratch
> mkfs.btrfs -m raid1 -d raid1 /dev/sdg /dev/sdh /dev/sdl

> WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
> WARNING! - see http://btrfs.wiki.kernel.org before using

> /dev/sdh is mounted

> ## Comment: Of course /dev/sdh is not mounted.
> mount |grep /dev/sdh
> root@lxc:~#

Two days ago I've seen a perhaps similar problem with an ext3 disk:

Connected via USB
running fsck (with some options)

   mount /dev/$disk $mountpoint

also told that the disk was mounted (it wasn't really).

Maybe some other operation (like - in this case - fsck) on/with the disk  
leads to this wrong message.

Viele Gruesse!
Helmut

Re: BTRFS thinks that a device is mounted

[Stephane CHAZELAS]

2011-10-21, 00:39(+03), Nikos Voutsinas:
[...]
> ## Comment: Of course /dev/sdh is not mounted.
> mount |grep /dev/sdh
> root@lxc:~#
[...]

Note that mount(8) uses /etc/mtab to find out what is mounted.
And if that file is not a symlink to /proc/mounts, the
information is not necessarily correct.

You can also have a look at /proc/mounts directly.

-- 
Stephane

Re: BTRFS thinks that a device is mounted

[Nikos Voutsinas]

> 2011-10-21, 00:39(+03), Nikos Voutsinas:
> [...]
>> ## Comment: Of course /dev/sdh is not mounted.
>> mount |grep /dev/sdh
>> root@lxc:~#
> [...]
>
> Note that mount(8) uses /etc/mtab to find out what is mounted.
> And if that file is not a symlink to /proc/mounts, the
> information is not necessarily correct.
>
> You can also have a look at /proc/mounts directly.
>

OK, I found what went wrong, however I am not sure whose fault this is and
If I am going to describe a case that is already known to this list.

To cut a long story short, the consecutive

root@btrfs:~# btrfs device add /dev/sdc /
root@btrfs:~# btrfs device delete /dev/sdc /

led to inconsistency between the contents of /proc/mounts and
/dev/disk/by-uuid (ie the mapping of UUIDs to devices).

Actually, "btrfs device add /dev/sdc /" changed the mapping of root's UUID
to point to /dev/sdc, and the system left it this way, even after the
"btrfs device delete /dev/sdc /". Thus I ended up with the following
situation.

root@btrfs:~# cat  /etc/mtab /proc/mounts |grep btrfs
/dev/sdd2 / btrfs rw 0 0
/dev/disk/by-uuid/cc8211ba-6f62-46d6-ba17-49250f06a941 / btrfs rw 0 0
root@btrfs:~#
root@btrfs:~# ls -l /dev/disk/by-uuid/cc8211ba-6f62-46d6-ba17-49250f06a941
lrwxrwxrwx 1 root root 9 Oct 22 16:13
/dev/disk/by-uuid/cc8211ba-6f62-46d6-ba17-49250f06a941 -> ../../sdc

Although this may be a udev issue, I think that the userspace tools of
btrfs should have handled this in a better way.

Nikos