Re: [Xen-users] Fw: coordinating VM snapshot with LVM snapshot.

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Jan 13, 2010 at 09:11:47AM +0200, Pasi Kärkkäinen wrote:
> On Tue, Jan 12, 2010 at 05:44:18PM -0800, J Nb wrote:
> > Anyone ?
> > 
> 
> Now that XenServer is opensourced as XCP (Xen Cloud Platform), you could
> take a look at the source.. 
> 
> I believe XenServer supports Windows VSS coordination for snapshots.
> "xe vm-snapshot-with-quiesce"
> 

Did you get anywhere with this? 

For a long time I've been thinking of writing some kind of 
"VSS" library/daemon for Linux.. similar to Microsoft Windows 
Volume Snapshot Services.

It could (and should) be called something different than VSS, 
but I'll just use that name for now..

The interface/usage could be something like:

        1. backup begins
        2. execute in the guest: "vss_control pre-snapshot xvda"
        3. take snapshot of the lvm volume having guest disk xvda
        4. execute in the guest: "vss_control post-snapshot xvda"
        5. do the actual backup from the lvm snapshot
        6. backup ends

This would have minimal downtime for the applications running in the guest.

pre-snapshot action would go through all the application specific pre-scripts, 
execute them, preparing the applications for taking the snapshot, and then 
it would flush the filesystem, kernel and disk caches as needed to prepare for
a consistent snapshot.

Preparing the application for taking the snapshot might mean something
like placing locks to database tables preventing disk writes from happening, 
or shutting down the application, or whatever the specific application requires.

post-snapshot action would again go through and execute all the application 
specific
post-scripts releasing the applications back to normal running state.

The application specific pre/post scripts could be stored like this:

        /etc/vss/scripts/xvda/001-mysql.pre
        /etc/vss/scripts/xvda/001-mysql.post

        /etc/vss/scripts/xvda/002-pgsql.pre
        /etc/vss/scripts/xvda/002-pgsql.post

        /etc/vss/scripts/xvda/003-my_custom_app.pre
        /etc/vss/scripts/xvda/003-my_custom_app.post

Or you could have "scripts/xvda2/*" if you have lvm volumes directly as
partitions to the guest. You get the idea.

Then the next step would be to create a daemon ("vssd") that listens for
requests from the network, and/or possibly from Xen dom0 through the hypervisor.

Or then the backup server might issue those pre-snapshot and post-snapshot 
commands through ssh.. many options how to implement it.

Does this make any sense? Comments?

-- Pasi

> 
> > 
> > --- On Sun, 1/3/10, J Nb <j_nwb@xxxxxxxxx> wrote:
> > 
> > > From: J Nb <j_nwb@xxxxxxxxx>
> > > Subject: coordinating VM snapshot with LVM snapshot.
> > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > Date: Sunday, January 3, 2010, 6:17 PM
> > > Hi
> > >    How to co-ordinate VM snapshots and disk
> > > snapshots ? Here is a sequence I am thinking of
> > > automating...
> > > 
> > >   1.Tell VM to sync (say via ssh or some special
> > > agent)
> > >      * This will do "sync" and any
> > > application specific hook to make it quiescent.
> > > 
> > >   2. Tell VM/Hypervisor to flush all disk buferes
> > > ?  (HOW ??? )
> > >      * Anything at this level?
> > > 
> > >   3. Pause the VM
> > > 
> > >   4. xm save -c (To save the VMs state in to
> > > checkpoint file.)
> > >       (This may take more time !)
> > > 
> > >   5. Take LVM snapshot for each disk. 
> > > 
> > >   6. Unpause the VM
> > > 
> > >   7. Application level resume if any.
> > >   
> > >   7. Use the snapshot to copy each disks ?!!!! (either
> > > dd or mount followed by cp/rsync)  (This can be big!
> > > Any optimization suggestions )   
> > > 
> > >   8. discard snapshot.
> > > 
> > > 
> > > Would the above sequence work ? 
> > > 
> > > Any suggestions on individual steps ?
> > > 
> > > Anything specific for PV and HVM virtual machines?
> > > 
> > > Is there a backdoor to qemu-dm console ?
> > > 
> > > Does anyone know how similar functionality is implemented
> > > in XenServer product or Virtual Iron or anyone else? 
> > > 
> > > -nb
> > > 
> > > 
> > >       
> > > 
> > 


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