snapshot doesn't flush dirty data

[jichenjc]

libvirt driver

Login a deployed VM and write some files, for example:
```
[lynn@host-172-26-105-43 ~]$ ls -lrt
...
-rw-rw-r--. 1 lynn lynn 677 Jun 29 06:30 sum
-rw-rw-r--. 1 lynn lynn 38 Jun 29 06:30 sum3.md5
[lynn@host-172-26-105-38 ~]$
```

then *right* after the file created, trigger the snapshot action of the VM
after that, deploy a new VM through the created image

found some files missing (e.g sum3.md5 file)
```
[lynn@host-172-26-105-43 ~]$ ls -lrt
...
-rw-rw-r--. 1 lynn lynn 648 Jun 29 06:30 sum
[lynn@host-172-26-105-43 ~]$
```

I checked the code and seems no flush operation in live or cold snapshot (cold seems suspend the VM only)
https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L2952

is it by design or best practice ?

[QingFeng Hao (johnnyhao)]

I encountered this problem too for both live and cold snapshot.
But if I shutdown the VM with 'virsh shutdown' or stop in the UI, the data can by synced to disk.
So I don't know why openstack doesn't use this in cold snapshot.

[Lee Yarwood (lyarwood)]

You need to use the QEMU guest agent within the guestOS to quiesce the disks fully during a snapshot:

https://docs.openstack.org/nova/latest/admin/configuration/hypervisor-kvm.html#guest-agent-support

[Launchpad Janitor (janitor)]

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