Openstack NOVA - Improve the time of file system freeze during live-snapshot

[Pierre Libeau]

Hello,

I'm working on a patch in nova to improve the time of file system freeze during live-snapshot on an instance with a local disk and I need your opinion about the solution I would propose.

My issue during the live migration is the duration of file system freeze on an instance with a big local disk. [1]
In my case instance have locally a disk (400Go) and the qemu-guest-agent is installed.
Nova process like that: [2]
dev = guest.get_block_device(disk_path)

1. guest.freeze_filesystems()
2. dev.rebase(disk_delta, copy=True, reuse_ext=True, shallow=True)
3. while not dev.is_job_complete() #wait for the end of mirroring (the issue is here, the waiting time depend on the size of the disk and the IOPS)
4. dev.abort_job()
5. guest.thaw_filesystems()

My proposition is to move the freeze after the end of mirroring and before the stop of mirroring. [3]
I have tried on an instance and the last written file on the fs corresponds to the end of the mirror.

Can you give me your opinion about the proposition?

[1] https://bugs.launchpad.net/nova/+bug/1939116
[2] https://review.opendev.org/plugins/gitiles/openstack/nova/+/2ffd9738602531e93495a1feca76bbb687c3e72c/nova/virt/libvirt/driver.py#3139
[3] https://review.opendev.org/c/openstack/nova/+/803713

Pierre