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

[Anthony Liguori]

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 (address@hidden) 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.