[Qemu-devel] [Bug 1587065] Re: btrfs qemu-ga - multiple mounts block fsfreeze

[Tim Rose]

I have a related Problem with a libvirt Guest running some Docker-
Containers.

address@hidden:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       7.9G  2.9G  4.6G  39% /
udev             10M     0   10M   0% /dev
tmpfs           794M  8.6M  785M   2% /run
tmpfs           2.0G  672K  2.0G   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup
none            7.9G  2.9G  4.6G  39% 
/var/lib/docker/aufs/mnt/8b510e9eb71523cf9afc723e11831a169c822b7251049f95ebed5e978b22184b
shm              64M     0   64M   0% 
/var/lib/docker/containers/3d5542423739bc1a4467ce5e15a3b82224d2d1b315af75d7d91a0cc04e1e7b07/shm
none            7.9G  2.9G  4.6G  39% 
/var/lib/docker/aufs/mnt/5d186a68a1fca2b58a440e6dbdf293853011a9526b819ac1dbcba7f91272535f
shm              64M     0   64M   0% 
/var/lib/docker/containers/a4355f5e2a66b25a564c5027f818b4385cd417f85d8aaf79700ca4086fb86e63/shm
none            7.9G  2.9G  4.6G  39% 
/var/lib/docker/aufs/mnt/e972052c347006cd8c79dc5bf9aaba07fdba2edeadc01e18e903d04ca4b05a86
none            7.9G  2.9G  4.6G  39% 
/var/lib/docker/aufs/mnt/e22995e962919ea50fcf7b4a5e52916e6839f76708d22e57e81de687af2f0dd0
shm              64M     0   64M   0% 
/var/lib/docker/containers/00e3490e37ded90e7421eb19e650cc684dec2b65a848a203c21212a549bf9e0e/shm
shm              64M     0   64M   0% 
/var/lib/docker/containers/631b37644a0ac106b7d7403c2fcc3760d4647cb96141915b0aa7ae78f98e4e05/shm
address@hidden:~# mount -l | grep /dev/vda2
address@hidden:~# mount -l | grep /dev/sda
/dev/sda1 on / type ext4 (rw,relatime,discard,errors=remount-ro,data=ordered)
/dev/sda1 on /var/lib/docker/aufs type ext4 
(rw,relatime,discard,errors=remount-ro,data=ordered)

I am not able to take snapshots:

error: internal error: unable to execute QEMU agent command 'guest-
fsfreeze-freeze': failed to freeze /: Device or resource busy

Thank you
Tim

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https://bugs.launchpad.net/bugs/1587065

Title:
  btrfs qemu-ga - multiple mounts block fsfreeze

Status in QEMU:
  New

Bug description:
  Having two mounts of the same device makes fsfreeze through qemu-ga 
impossible.
  address@hidden:/# mount -l | grep /dev/vda2
  /dev/vda2 on / type btrfs (rw,relatime,space_cache,subvolid=257,subvol=/@)
  /dev/vda2 on /home type btrfs 
(rw,relatime,space_cache,subvolid=258,subvol=/@home)

  Having two mounts is rather common with btrfs, so the feature fsfreeze
  is unusable on these systems.

  
  Below more information about how we encountered this issue...

  Message send to address@hidden:

  Message 1:
  ----------
  I use external snapshots to backup my guests. I use the 'quiesce' option to 
flush and frees the guest file system with the qemu guest agent.

  With the exeption of one guest, this procedure works fine. On the 'unwilling' 
guest, I get this error message:
  "ERROR 2016-05-25 00:51:19 | T25-bakVMSCmsrvVH2 | fout: internal error: 
unable to execute QEMU agent command 'guest-fsfreeze-freeze': failed to freeze 
/: Device or resource busy"

  I don't think this is not some sort of time-out error, because
  activation of the fsfreeze and the error message happen immediately
  after each other:

  $ grep qemu-ga syslog.1
  May 25 00:51:19 CmsrvMTA qemu-ga: info: guest-fsfreeze called

  This is the only entry of the qemu guest agent in syslog.

  $ sudo virsh version
  Compiled against library: libvirt 1.3.1
  Using library: libvirt 1.3.1
  Gebruikte API: QEMU 1.3.1
  Draaiende hypervisor: QEMU 2.5.0

  $ virsh qemu-agent-command CmsrvMTA '{"execute": "guest-info"}'
  {"return":{"version":"2.5.0", ... 
,{"enabled":true,"name":"guest-fstrim","success-response":true},{"enabled":true,"name":"guest-fsfreeze-thaw","success-response":true},{"enabled":true,"name":"guest-fsfreeze-status","success-response":true},{"enabled":true,"name":"guest-fsfreeze-freeze-list","success-response":true},{"enabled":true,"name":"guest-fsfreeze-freeze","success-response":true},
 ... }

  For making an external snapshot, I use this command:
  $ virsh snapshot-create-as --domain CmsrvMTA sn1 --disk-only --atomic 
--quiesce --no-metadata --diskspec vda,file=/srv/poolVMS/CmsrvMTA.sn1

  Piece of reply 1:
  -----------------
  I have encountered this before. Some operating systems
   may have bind-mounts that let a device appear multiple times in the mount 
list. Unfortunately the guest agent is not smart enough to consider a device 
that has been frozen as succesfull and keep going. This causes this specific 
error.

  Piece of reply 2:
  -----------------
  Ok, that seems to be it.

  I’ve got the ‘/’ and ‘/home’ on the same device formatted as btrfs on
  two separate sub volumes.

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