Re: libvirt: Removing RBD volumes with snapshots, auto purge or not?

[Wido den Hollander <wido@42on.com>]

On 08/20/2013 05:43 PM, Andrey Korolyov wrote:
> On Tue, Aug 20, 2013 at 7:36 PM, Wido den Hollander <wido@42on.com> wrote:
>> Hi,
>>
>> The current [0] libvirt storage pool code simply calls "rbd_remove" without
>> anything else.
>>
>> As far as I know rbd_remove will fail if the image still has snapshots, you
>> have to remove those snapshots first before you can remove the image.
>>
>> The problem is that libvirt's storage pools do not support listing
>> snapshots, so we can't integrate that.
>>
>> Libvirt however has a flag you can pass down to tell you want the device to
>> be zeroed.
>>
>> The normal procedure is that the device is filled with zeros before actually
>> removing it.
>>
>> I was thinking about "abusing" this flag to use it as a snap purge for RBD.
>>
>> So a regular volume removal will call only rbd_remove, but when the flag
>> VIR_STORAGE_VOL_DELETE_ZEROED is passed it will purge all snapshots prior to
>> calling rbd_remove.
>>
>> Another way would be to always purge snapshots, but I'm afraid that could
>> make somebody very unhappy at some point.
>>
>> Currently "virsh" doesn't support flags, but that could be fixed in a
>> different patch.
>>
>> Does my idea sound sane?
>>
>> [0]:
>> http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/storage/storage_backend_rbd.c;h=e3340f63f412c22d025f615beb7cfed25f00107b;hb=master#l407
>>
>> --
>> Wido den Hollander
>> 42on B.V.
>
> Hi Wido,
>
>
> You had mentioned not so long ago the same idea as I had about a year
> and half ago about placing memory dumps along with the regular
> snapshot in Ceph using libvirt mechanisms. That sounds pretty nice
> since we`ll have something other than qcow2 with same snapshot
> functionality but your current proposal does not extend to this.

Correct, since this is about the storage pool support, that is something 
completely different.

> Placing custom side hook seems much more expandable than putting snap
> purge into specific flag.
>

My proposal is a bit selfish, since I'm running into this with 
CloudStack. CloudStack now has a work-around for RBD since images could 
still have snapshots where other storage types are handled by libvirt.

I want to have it all handled by libvirt.

Wido

>>
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>> Skype: contact42on
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-- 
Wido den Hollander
42on B.V.

Phone: +31 (0)20 700 9902
Skype: contact42on