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

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

On 08/20/2013 11:12 PM, Josh Durgin wrote:
> On 08/20/2013 08:36 AM, Wido den Hollander 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's storage pools don't have any concept of snapshots, which is
> the real problem. Ideally they would have functions to at least create,
> list and delete snapshots (and probably rollback and create a volume
> from a snapshot too).
>
>> 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.
>
> I don't think we should reinterpret the flag like that. A new flag
> for that purpose could work, but since libvirt storage pools don't
> manage snapshots at all right now I'd rather CloudStack delete the
> snapshots via librbd, since it's the service creating them in this case.
> You could see what the libvirt devs think about a new flag though.
>

CloudStack currently does that: 
https://git-wip-us.apache.org/repos/asf?p=cloudstack.git;a=blob;f=plugins/hypervisors/kvm/src/com/cloud/hypervisor/kvm/storage/LibvirtStorageAdaptor.java;h=719a03d60d07e2fd76b779e02a052e7e18a66877;hb=master#l702

I'll ask the libvirt guys about this later, but there is some big 
latency on their mailinglist and I have two outstanding questions. I'll 
wait for those before starting about this.

>> Another way would be to always purge snapshots, but I'm afraid that
>> could make somebody very unhappy at some point.
>
> I agree this would be too unsafe for a default. It seems that's what
> the LVM storage pool does now, maybe because it doesn't expect
> snapshots to be used.
>
>> Currently "virsh" doesn't support flags, but that could be fixed in a
>> different patch.
>
> No backend actually uses the flags yet either.
>

True, but support is in there for usage of such flags. So why not use it?

> Josh


-- 
Wido den Hollander
42on B.V.

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