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?
>
> > >
> > > 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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