Re: [Xen-users] snapshots on xen

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Apr 12, 2010 at 09:31:59AM +0200, Thomas Halinka wrote:
> Hi Pasi,
> 
> Am Montag, den 12.04.2010, 09:20 +0300 schrieb Pasi Kärkkäinen:
> > On Sat, Apr 10, 2010 at 08:58:59AM -0400, James Pifer wrote:
> > > On Sat, 2010-04-10 at 13:55 +0300, Pasi Kärkkäinen wrote:
> > > > On Fri, Apr 09, 2010 at 08:57:16PM -0400, James Pifer wrote:
> > > > > Hi all. I'm using sles11 for my xen servers. I would really like to be
> > > > > able to do hot snapshots of disk AND memory, and then have these
> > > > > snapshots backed up to tape or off site for disaster recovery. I'm
> > > > > thinking this would be done weekly or monthly, not nightly.
> > > > > 
> > > > > Two questions:
> > > > > 
> > > > > 1) Is this even possible? 
> > > > >
> > > > 
> > > > 1) xm save <guest> <guest>.save
> > > > 2) save a copy of the guest disks
> > > > 3) save a copy of the <guest>.save file
> > > > 4) xm restore <guest>.save
> > > >  
> > > 
> ..........
> > When the guest is saved/stopped, you can take a backup of the disks,
> > and store the backup with the state/save-file.
> 
> how does this behave with blktap2? i read that blktap2 passes all disk
> I/O-requests from VMs to the userspace deamon through a character
> device.
> 
> As i read the release-notes correctly we should get a consistent FS,
> without "xm save" just through using blktap2/vhd?
> 

Depends what you mean with 'consistent'. 

If you want to do a disk snapshot online then you always need to coordinate 
the snapshot with the guest OS/kernel/apps - the guest needs to have 
the apps in a consistent state and all the buffers flushed when you take the 
disk snapshot.

Windows provides VSS framework for this, but there's nothing general in Linux 
for this.
And also you need to coordinate that stuff with the snapshot, have the timing 
correct.

So, even if you used blktap2/vhd, you'd have to trigger and coordinate the 
'prepare apps and flush caches'
in the guest to happen at the correct time for the disk snapshot to be 
consistent.

XenServer/XCP has method for this, through the Citrix windows PV drivers.
So yeah.. blktap2 is just a part of the solution. You need more to actually do 
it properly. 

So "xm save" method is easier.. 

-- Pasi


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