[libvirt] A couple of questions on snapshot-create/ snapshot-create-as

[Eric Blake]

On 04/30/2012 06:17 AM, Kashyap Chamarthy wrote:
> Hi (Eric?),
> 
> I got a couple of questions for 'snapshot-create' / 'snapshot-create-as' :
> 
> 
> 
> 1/ can '--disk-only' option (when used with virsh snapshot-create-as) be run on an offline
> guest and generate an 'external snapshot' ?

Not yet, but it's on my list of things to add.

> Or a running guest is needed?

For now, this is true.

> 
> 
> 2/ Can anyone point me to a bit more verbose info on these below options while creating
> snapshots?
> 
>  --quiesce        quiesce guest's file systems
>  --atomic         require atomic operation
>  --reuse-external reuse any existing external files

http://libvirt.org/html/libvirt-libvirt.html#virDomainSnapshotCreateXML

(Yuck - our online doc conversion loses paragraph breaks, and isn't
understanding how to mark up parameter names).

--quiesce maps to VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE, which requests
that if the guest has a guest agent running, then connect to that agent
and request that the file systems be frozen before taking the snapshot
(that way, you can then directly boot the snapshot without having to run
fsck).

--atomic maps to VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC.  If you are using
qemu 1.0 or earlier, this flag will fail unless you only snapshot a
single disk; if you are using qemu 1.1 or later, this flag will ensure
that the qemu monitor command 'transaction' is used to ensure an atomic
snapshot.  Omitting the flag has no change of behavior on qemu 1.1, but
on qemu 1.0 or earlier, the snapshot is attempted, but if you fail
partway through, then you may be left with a domain where half the disks
had a snapshot and the other half did not.

--reuse-external maps to VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT, which
states that rather than having libvirt create the snapshot file from
scratch, you will pre-create the file.  This is useful for applications
like VDSM that want the snapshot to be pre-created with a relative
backing file name in the qcow2 metadata (libvirt would create it with an
absolute backing file name).

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

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