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Re: [libvirt-users] Managing Live Snapshots with Libvirt 1.0.1

On 02/04/2013 02:37 PM, Andrew Martin wrote:
> Thanks for this clarification. When taking live snapshots, what does
> the --quiesce option do exactly? Does it call a sync on the guest to
> make sure the disks are in a consistent state? If so, does it only work
> for specific drivers, e.g. VirtIO disks? Would it be best to use this
> option when taking live snapshots to ensure that the snapshot disk is
> consistent?

--quiesce says to send a message on the qemu-guest-agent to tell the
guest to get its file systems into a consistent state.  For it to be
useful, you have to:
1. trust your guest (anything that involves guest cooperation can
backfire if you don't trust the guest)
2. have the qemu-guest-agent channel wired up in the guest XML (there's
a request to make this easier to do; but for now,
http://libvirt.org/formatdomain.html#elementCharChannel documents how to
set up org.qemu.guest_agent.0)
3. have the guest agent actually running in the guest (so far, it has
been ported to at least Windows and Linux, but as recently as Fedora 18,
I raised a bug that the default live image build was not installing the
guest agent by default. If your guest is running some other OS, then you
will probably have to contribute patches to the qemu list to port the
guest agent to your choice of guest)

Additionally, if you use the guest agent from (not yet released) qemu
1.4 or later, there was a recent addition in upstream qemu that added
hooks so you could add additional wrapper actions (such as a database
quiesce command) around the agent commands that freeze and rethaw all
file systems, for even more stability in the snapshot image.

Using --quiesce is therefore optional; if you use it, you are more
likely to be able to boot your guest from the snapshot point in time
without having inconsistent file systems; if you do not use it, you are
more likely to have to run fsck or similar to recover files that were in
the middle of being modified at the time of the snapshot (since a
disk-only snapshot doesn't include guest RAM state).

Meanwhile, with libvirt 1.0.1 and later, an external snapshot including
RAM does not support the --quiesce option, for two reasons: 1. --quiesce
requires that the guest be running in order to freeze its own file
systems, but taking a snapshot of memory at the same time as a snapshot
of the disks requires pausing the guest (even if the downtime is limited
to the sub-second range by use of the --live flag). 2. if you have the
RAM state available, then you have not lost any in-flight I/O operations
that were in the guest RAM, so it is no longer quite as essential as it
was for disk-only snapshots to get the disks into a stable state.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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