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


From:  Christian Ehrhardt 
Subject: [Qemu-devel] [Bug 1587065] Re: btrfs qemu-ga - multiple mounts block fsfreeze
Date: Thu, 28 Jun 2018 11:13:42 -0000

Steps to reproduce

uvt-simplestreams-libvirt --verbose sync --source 
http://cloud-images.ubuntu.com/daily arch=amd64 release=xenial label=daily
uvt-kvm create --password ubuntu x-freeze arch=amd64 release=xenial label=daily

Add this to the guest definition and restart it
<channel type='unix'>
   <source mode='bind'/>
   <target type='virtio' name='org.qemu.guest_agent.0'/>
</channel>

>From the host freeze and thaw it:
$ virsh domfsfreeze x-freeze
$ virsh domfsthaw x-freeze

# The above works

In the Guest then add a bind mount to trigger the issue
$ sudo mkdir /mnt/test
$ sudo mount -o bind / /mnt/test/

Then again try to freeze/unfreeze
$ virsh domfsfreeze x-freeze
error: Unable to freeze filesystems
error: internal error: unable to execute QEMU agent command 
'guest-fsfreeze-freeze': failed to freeze /: Device or resource busy


I installed a fix from my PPA [1] and with that it works just fine now.
Proposing an MP for review by the team, to afterwards move on into SRU 
processing.

[1]: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/3310

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

Title:
  btrfs qemu-ga - multiple mounts block fsfreeze

Status in QEMU:
  Fix Released
Status in qemu package in Ubuntu:
  Fix Released
Status in qemu source package in Xenial:
  Confirmed

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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