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[nova][glance][cinder] How to do consistent snapshots with quemu-guest-agent


Hello,


Working me through documentation and articles I am totally lost on the matter.

All I want to know is:

- if issueing "openstack snapshot create ...."

- if klicking "create Snaphost" in Horizon for an instance

will secure a consistent snapshot (of all volumes in question).
With "consistent", I mean that all the data in memory are written to the disc before starting a snapshot.


I hope someone can clear up, if using the setup described in the following is sufficient to achieve this goal or if I have to do something in addition.


If you have any question I am eager to answer as fast as possible.


Setup:


We have a Stein-based OpenStack deployment with cinder backed by ceph.

Instances are created with cinder volumes. Boot volumes are based on an image having the properties:

- hw_qemu_guest_agent='yes'
- os_require_quiesce='yes'


The image is ubuntu 16.04 or 18.04 with quemu-guest-agent package installed and service running (no additional configuration besides distro-default):

qemu-guest-agent.service - LSB: QEMU Guest Agent startup script
   Loaded: loaded (/etc/init.d/qemu-guest-agent; bad; vendor preset: enabled)
   Active: active (running) since Wed 2019-08-14 07:42:21 UTC; 9min ago
     Docs: man:systemd-sysv-generator(8)
   CGroup: /system.slice/qemu-guest-agent.service
           â??â??2300 /usr/sbin/qemu-ga --daemonize -m virtio-serial -p /dev/virtio-ports/org.qemu.guest_agent.0

Aug 14 07:42:21 ulthwe systemd[1]: Starting LSB: QEMU Guest Agent startup script...
Aug 14 07:42:21 ulthwe systemd[1]: Started LSB: QEMU Guest Agent startup script.

I can see the socket on the compute node and send pings successfully:

~# ls /var/lib/libvirt/qemu/*.sock
/var/lib/libvirt/qemu/org.qemu.guest_agent.0.instance-0000248e.sock
root at pcevh2404:~# virsh qemu-agent-command instance-0000248e '{"execute":"guest-ping"}'
{"return":{}}


I can also send freeze and thaw successfully:

~# virsh qemu-agent-command instance-0000248e '{"execute":"guest-fsfreeze-freeze"}'
{"return":1}

~# virsh qemu-agent-command instance-0000248e '{"execute":"guest-fsfreeze-thaw"}'
{"return":1}

Sending a simple write (echo "bla" > blub.file) in the "frozen" state will be blocked until "thaw" as expected.

Best regards


Ralf T.
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