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Re: [libvirt] libvirt + kvm external snapshots: use case?



On 07/29/2012 06:08 PM, Kashyap Chamarthy wrote:
> On 07/16/2012 01:33 AM, Cole Robinson wrote:
>> Forgive my ignorance, but I'm trying to fill in some details for a Fedora
>> feature page:
>>
>> https://fedoraproject.org/w/index.php?title=Features/Virt_Live_Snapshots
>>
>> Yet things aren't working quite as expected in my tests. I'm trying to think
>> of this from an end user use case perspective. For example
>>
>> - I want to test some changes to my vm, and either throw them away, or commit
>> them to my base disk image.
>> - I want to have a base disk image, but have multiple snapshots with various
>> mutually exclusive VM changes in each, like one snapshot with libvirt 0.7.0
>> installed, another with 0.8.0 installed. I want to switch between them
>>
>> (maybe those assumptions are off base to begin with)
>>
>> So for a running guest, I do:
>>
>> virsh snapshot-create-as myvm snapshot1 "snapshot1 description" --disk-only
>> virsh snapshot-create-as myvm snapshot2 "snapshot2 description" --disk-only
> 
> 
> Another possible way is to specify the device name explicitly using --diskspec
> 
> =====================================================================
> root ~$ virsh domblklist f17test --details
> Type       Device     Target     Source
> ------------------------------------------------
> file       disk       vda        /var/lib/libvirt/images/f17test.qcow2
> =====================================================================
> root ~$ virsh snapshot-create-as f17test cleanf17 "cleanf17-box" --diskspec
> vda,file=/var/lib/libvirt/images/snap-f17test.qcow2 --disk-only
> Domain snapshot cleanf17 created
> root ~$
> =====================================================================
> 
>>
>> I make some changes that I want to discard, so I try and switch back to the
>> older vm and want to switch back to an older snapshot state, odo:
>>
>>   virsh snapshot-revert myvm snapshot1
>>
>> But I get: 'revert to external disk snapshot not supported yet'. Hmm, that
>> kind of invalidates all my use cases then. Additionally I don't see any API
>> option for merging snapshots, to fold my changes in snapshot2 into parent
>> snapshot1 or similar. I tried using snapshot-delete, but that also fails with
>> external disk snapshots: 'deletion of 1 external disk snapshots not supported yet'
>>
>> Additionally, there's the --atomic option to snapshot-create-as, but it fails
>> for guests with more than 1 snapshotable disks: 'atomic live snapshot of
>> multiple disks is unsupported'. But isn't that what some of the recent qemu
>> additions explicitly supposed to support (that's the impression I got from jtc)?
>>
>> So am I missing anything? If not, are things just not fully wired up or are
>> there missing chunks of functionality? And what's the use case for the current
>> impl of disk snapshoting (while I've read around and found lot's of info about
>> the implementation side of things, not too much about intended uses)?
> 
>>From a previous discussion with Eric, with external snapshots the ‘original qcow2 file’
> will be in a ‘read-only’ saved state, and the new qcow2 file(which will be generated once
> snapshot is created) will be the *delta* for the changes. So, all the changes will now be
> written to this delta file. External Snapshots appear to be useful for performing
> backups(though I'm yet to try those). Also, external snapshot creates a qcow2 file with
> the original file as its backing image, and the backing file can be /read/ in parallel
> with the running qemu.
> 
> (side note: though, if we're not using --disk-only, then offline snapshots appear to be
> faster.)
> 
> I'm also currently trying out different possibilities of external snapshots and trying to
> explore and understand proper use-cases. I'd be curious to hear more on this.  

I missed to read Eric's informative reply. Disregard me.

> I'll will
> post my notes and participate in the above test day.
> 
> Here's a slightly old blog post I made w/ snapshots -
> kashyapc.wordpress.com/2011/10/04/snapshotting-with-libvirt-for-qcow2-images/
> 
>>
>> Thanks,
>> Cole
>>
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>>
> 
> 


-- 
/kashyap



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