[libvirt-users] Problem with the use of domfsfreeze mountpoint option

[Eric Blake]

On 11/12/2014 10:24 AM, Michal Privoznik wrote:
>> What version of qemu-guest-agent is running in the guest?
>> qemu-guest-agent doesn't support per-mountpoint freezing until the
>> introduction of guest-fsfreeze-freeze-list in qemu 2.2 (still
>> unreleased).
>>
>>>
>>> --Upgraded libvirt to 1.2.10, but that also didn't solve the problem.
>>>
>>> Am i missing something over here?
>>> Any help would be greatly appreciated.
>>
>> I wonder if you may have uncovered a libvirt bug.  If the guest agent is
>> not capable of supporting per-mount freezing (because the agent is too
>> old), the command should fail rather than blindly freezing everything.
>> But to know for sure, it would be good to find the log messages for the
>> actual agent commands issued by libvirt, to make sure we were actually
>> trying to freeze just a single mount point.
> 
> Well, even though I agree I don't see way to achieve that. I mean, if
> libvirt would probe for qemu-ga commands/capabilities, by the time it
> makes a decision guest agent may have been downgraded, crashed,
> whatever. There's been some discussion on this topic (unfortunately I
> don't recall where currently). This is where guest agent is different to
> the monitor tremendously.

Well, libvirt should still be able to at a minimum detect an error for
trying an unsupported command (as it must use a different command when
freezing specific mountpoints than for freezing all disks).

> 
> Although, I see a way that we could get something reasonable here. If
> qemu would tell us whenever somebody (dis-)connects (from)to the virtio
> channel. That way we could query the qemu-ga capabilities and make good
> decisions. And whenever we see a disconnect, we may just forget the
> qemu-ga capabilities and claim guest agent unresponsive (instead of this
> ping algorithm I'd came up with).

Yes, qemu now provides that, as of qemu 2.1.  It is the VSERPORT_CHANGE
event that fires whenever the guest opens or closes its connection to
the channel.  And yes, we have more than one reason why we should wire
up libvirt to track when that event happens - we ALSO have people
requesting that we expose the information to management apps as a
libvirt event.

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

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