libvirt users - May 2016 - Issue with fsfreeze with qemu agent.

Hi,

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.

The difference of this guest in compresence with other, is that this guest runs
postfix + amavis + spammassasin + clamav

Any help would be appreciated.

More info:

$ 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

Greetings,

Dominique.