Re: [Qemu-devel] Re: KVM call agenda for Oct 19

Anthony Liguori
2010-10-19T13:18:10Z

On 10/19/2010 08:03 AM, Avi Kivity wrote:
> On 10/19/2010 02:58 PM, Dor Laor wrote:
>> On 10/19/2010 02:55 PM, Avi Kivity wrote:
>>> On 10/19/2010 02:48 PM, Dor Laor wrote:
>>>> On 10/19/2010 04:11 AM, Chris Wright wrote:
>>>>> * Juan Quintela (quintela@redhat.com) wrote:
>>>>>>
>>>>>> Please send in any agenda items you are interested in covering.
>>>>>
>>>>> - 0.13.X -stable handoff
>>>>> - 0.14 planning
>>>>> - threadlet work
>>>>> - virtfs proposals
>>>>>
>>>>
>>>> - Live snapshots
>>>> - We were asked to add this feature for external qcow2
>>>> images. Will simple approach of fsync + tracking each requested
>>>> backing file (it can be per vDisk) and re-open the new image would
>>>> be accepted?
>>>> - Integration with FS freeze for consistent guest app snapshot
>>>> Many apps do not sync their ram state to disk correctly or frequent
>>>> enough. Physical world backup software calls fs freeze on xfs and
>>>> VSS for windows to make the backup consistent.
>>>> In order to integrated this with live snapshots we need a guest
>>>> agent to trigger the guest fs freeze.
>>>> We can either have qemu communicate with the agent directly through
>>>> virtio-serial or have a mgmt daemon use virtio-serial to
>>>> communicate with the guest in addition to QMP messages about the
>>>> live snapshot state.
>>>> Preferences? The first solution complicates qemu while the second
>>>> complicates mgmt.
>>>
>>> Third option, make the freeze path management -> qemu -> virtio-blk ->
>>> guest kernel -> file systems. The advantage is that it's easy to
>>> associate file systems with a block device this way.
>>
>> OTH the userspace freeze path already exist and now you create
>> another path.
>
> I guess we would still have a userspace daemon; instead of talking to
> virtio-serial it talks to virtio-blk. So:
>
> management -> qemu -> virtio-blk -> guest driver -> kernel fs
> resolver -> daemon -> apps
>
> Yuck.

Yeah, in Windows, I'm pretty sure the freeze API is a userspace
concept. Various apps can hook into it to serialize their state.

At the risk of stealing Mike's thunder, we've actually been working on a
simple guest agent exactly for this type of task. Mike's planning an
RFC for later this week but for those that are interested the repo is at
http://repo.or.cz/w/qemu/mdroth.git

Regards,

Anthony Liguori
>
>> What about FS that span over LVM with multiple drives? IDE/SCSI?
>
> Good points.
>