[libvirt] A question for external snapshots with flag“VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE”

[Eric Blake]

please don't include libvir-list-owner in your mails.  Also, it would be
nice if you could convince your mailer to wrap long lines.

On 02/28/2017 06:37 AM, lizhengui wrote:
> Hello, I got a question here. When we create consistent active
external snapshots with flag “VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE” , the
qemuDomainSnapshotFSFreeze will be called firstly to freeze all
filesystems in vm, and then create snapshots. For windows vm, freezing
filesystems used by vss service. If IO pressure in vm is too big or
internal error happened in vss , freezing filesytems will failed , and
qemuDomainSnapshotFSFreeze returns 0 which meaning that no filesystems
are frozen.

That sounds like a bug in the qga implementation for freezing file
systems on Windows - if the system is not frozen, qga should return
failure, not 0.  Since libvirt doesn't maintain qga, this should
probably be reported to the qemu folks.

> In the function qemuDomainSnapshotCreateActiveExternal, libvirt
creates external snapshots all the same in such a situation that
qemuDomainSnapshotFSFreeze returns 0, but the created snapshots are not
consistent snapshots in such a situation . So shouldn't we abandon
creating snapshots and goto cleanup in the situation that
qemuDomainSnapshotFSFreeze returns 0?

Yes, even if qga behavior is patched, libvirt should probably figure out
if it is feasible to work around existing broken behavior.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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