Re: [Qemu-devel] [PATCH v4 08/10] qemu-ga: call Windows VSS requester in fsfreeze command handler

[Eric Blake]

On 07/01/2013 07:29 AM, Laszlo Ersek wrote:
> some comments below
> 
> On 06/06/13 17:06, Tomoki Sekiyama wrote:
>> Support guest-fsfreeze-freeze and guest-fsfreeze-thaw commands for Windows
>> guests. When fsfreeze command is issued, it calls the VSS requester to
>> freeze filesystems and applications. On thaw command, it again tells the VSS
>> requester to thaw them.
>>

>>  int64_t qmp_guest_fsfreeze_thaw(Error **err)
>>  {
>> +    int i;
>> +
>> +    if (!vss_initialized()) {
>> +        error_set(err, QERR_UNSUPPORTED);
>> +        return 0;
>> +    }
>> +
>> +    qga_vss_fsfreeze_thaw(&i, err);
> 
> I wonder how libvirt interprets a failure to thaw -- does it expect
> filesystems to remain frozen? (CC'ing Eric.)

On failure to thaw, libvirt marks the virDomainSnapshotCreateXML() API
call as failed, and logs the failure returned from the guest agent; but
beyond that, there really isn't anything further that libvirt can
attempt.  Guest freeze/thaw is a best-effort attempt, and we don't
really have any clean way to recover from a non-cooperative guest (ie.
it's no different from a Linux guest managing to kill off the guest
agent daemon in the guest at the wrong time).  In short, a failed
snapshot request has no guarantees on whether the guest is frozen or
thawed, and libvirt currently has no additional support to try a thaw in
isolation (although there is an intentionally unsupported 'virsh
qemu-agent-command' for attempting recovery manually).

> ... This patch made me look at ga_command_state_cleanup_all().
> Apparently the POSIX flavor thaws filesystems if qemu-ga exits before a
> thaw command over QMP; see guest_fsfreeze_cleanup() and
> ga_command_state_init(). Do you think something similar would be useful
> for the Windows flavor as well?

I agree that it is better to try and clean up a thaw on any error case
(including the error of qemu-ga exiting without seeing a QMP thaw
request), if only to minimize the chance of failure leaving the system
wedged with no chance of further recovery attempts.

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

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