WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

Re: [Xen-users] snapshots on xen

On Mon, Apr 12, 2010 at 10:40:01AM +0200, Thomas Halinka wrote:
> Hi Pasi,
> 
> Am Montag, den 12.04.2010, 10:39 +0300 schrieb Pasi Kärkkäinen:
> > 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'. 
> 
> 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.

> > 
> > 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. 

XCP also has/uses blktap2.

> > 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. 
> 
> Hmm, ok - just found it in the docs, too
> 
> ..
> 
>     114 Snapshots:
>     115 
>     116 Pausing a guest will also plug the corresponding IO queue for blktap2
>     117 devices and stop blktap2 drivers.  This can be used to implement a
>     118 safe live snapshot of qcow and vhd disks.  An example script "xmsnap"
>     119 is shown in the tools/blktap2/drivers directory.  This script will
>     120 perform a live snapshot of a qcow disk.  VHD files can use the
>     121 "vhd-util snapshot" tool discussed above.  If this snapshot command is
>     122 applied to a raw file mounted with tap:tapdisk:AIO, include the -m
>     123 flag and the driver will be reloaded as VHD.  If applied to an already
>     124 mounted VHD file, omit the -m flag.
>     125 
> 

See:
xe vm-snapshot-with-quiesce

That coordinates the backup with Windows guests using VSS, so that the 
applications
are in a known/good state, and the filesystem/kernel has flushed all the 
buffers/caches.

> So my next question is:
> 
> blktap2/vhd seems great to do snapshots and clones and will have 
> future-support for 
> thin-provisionig (like pre-allocation), but are there any advantages over lvm 
> at the moment?
> 

I haven't done any benchmarks myself, but the blktap2 snapshots might be faster.
Feel free to try and report back.

And of course blktap2 has support for the VHD format.

-- Pasi


_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users

<Prev in Thread] Current Thread [Next in Thread>