Re: [Xen-users] snapshots on xen

[Thomas Halinka <lists@xxxxxxxxx>]

Hi again,

Am Montag, den 12.04.2010, 12:01 +0300 schrieb Pasi KÃrkkÃinen: 
> 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?

> 
> > > 
> > > 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?

> 
> 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

yeah, i know about that

> 
> 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
> 
> 
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