Re: [Xen-users] snapshots on xen

[Thomas Halinka <lists@xxxxxxxxx>]

Hi Pasi,

Am Montag, den 12.04.2010, 10:39 +0300 schrieb Pasi KÃrkkÃinen:
> 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 KÃrkkÃinen:
> > > On Sat, Apr 10, 2010 at 08:58:59AM -0400, James Pifer wrote:
> > > > On Sat, 2010-04-10 at 13:55 +0300, Pasi KÃrkkÃinen 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

> 
> 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.

> 
> 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.

> 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 

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?

> So "xm save" method is easier.. 
> 
> -- Pasi
> 


thanks,

thomas



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