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



On 04/30/2012 06:48 PM, Eric Blake wrote:
> 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).

I'm assuming you're talking about this 'agent' --
http://wiki.qemu.org/Features/QAPI/GuestAgent

> 
> --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).
> 


Thanks Eric, for the clarification and detailed info.

-- 
/kashyap


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