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

From: Christian Theune
Subject: [Qemu-devel] [Bug 1587065] Re: btrfs qemu-ga - multiple mounts block fsfreeze
Date: Mon, 30 May 2016 13:56:49 -0000

I'm the responder from the Qemu list. I reviewed that C code a while ago
when I stumbled over it. If someone helps me through the Qemu patch
acceptance process then I'm willing to provide a patch.

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  btrfs qemu-ga - multiple mounts block fsfreeze

Status in QEMU:

Bug description:
  Having two mounts of the same device makes fsfreeze through qemu-ga 
  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 

  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", ... 
 ... }

  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 

  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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