Re: [Xen-users] snapshots on xen

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Apr 12, 2010 at 04:40:27PM +0200, Thomas Halinka wrote:
> Hi again,
> 
> Am Montag, den 12.04.2010, 12:01 +0300 schrieb Pasi Kärkkäinen: 
> > On Mon, Apr 12, 2010 at 10:40:01AM +0200, Thomas Halinka wrote:
> ..... 
> > > > Depends what you mean with 'consistent'. 
> > > 
> > > consistent means, that all buffer-caches and IO-queues were
> > > written/flushed to disks
> > > 
> > 
> > That requires coordination with the guest OS. It cannot be done
> > only from the hypervisor/dom0.
> > 
> > Doing the snapshot only from dom0 without any coordination with the guest 
> > is called 'crash consistent' snapshot. It's roughly the same as if you
> > pulled the power plug from a physical computer.
> > 
> > You'll get fsck when you restore that kind of snapshot and start the guest.
> > And some applications might be in a bad state.
> > 
> > > > 
> > > > 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.
> > > 
> > > Yeah, but i understood, that xen 4.0 implemented "some magic" around
> > > this topic.
> > > 
> > 
> > Xen 4.0 has better tools for doing the dom0/hypervisor side of it,
> > but you still need to do the guest OS side yourself, 
> > or by using some scripts together with the Xen tools.
> 
> are there any examples for such triggering from within xen tools?
> 

Well.. usually you'd write your own script, which does the magic, 
and uses xen tools as a part of the script.


> > 
> > > > 
> > > > 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.
> > > 
> > > The XEN-Datasheet (http://www.xen.org/files/Xen_4_0_Datasheet.pdf) says:
> > > 
> > > ....
> > > Blktap2
> > > A new virtual hard disk (VHD)
> > > implementation delivers high
> > > performance VM snapshots and
> > > cloning features as well as the
> > > ability to do live virtual disk
> > > snapshots without stopping a VM
> > > process.
> > > 
> > > So i thought it just works, eg through some kernel-hacking in pvops, or 
> > > whatever.
> > > 
> > 
> > Yeah well.. like said, that's only the hypervisor/dom0 bits of it.
> > Remember Xen 4.0 is just the core hypervisor, like an engine for a car.
> > 
> > XCP implements the 'other' needed bits (vm-snapshot-with-quiesce) 
> > through Citrix Windows PV drivers. 
> 
> is something equal planned for OS-XEN, too? Something on the road-map or
> some outstanding projects?
> 

Not that I know of.

I've been thinking of starting a "VSS for Linux" project for a long time,
just too busy with everything to actually do it right now :(

That's something we would really much need..

-- Pasi


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