Re: Ceph and KVM live migration

[Josh Durgin <josh.durgin@inktank.com>]

On 06/30/2012 07:15 PM, Vladimir Bashkirtsev wrote:
> On 01/07/12 10:47, Josh Durgin wrote:
>> On 06/30/2012 05:42 PM, Vladimir Bashkirtsev wrote:
>>> Dear all,
>>>
>>> Currently I testing KVMs running on ceph and particularly testing recent
>>> cache feature. Performance is of course vastly improved but still have
>>> occasional KVM hold ups - not sure who is at blame ceph of KVM. But I
>>> will deal with it later. Right now I've got myself a question which I
>>> could not get answered myself: if I do live migration of KVM while there
>>> some uncommitted data in ceph cache will this cache be committed prior
>>> cut-over to another host? Reading through the list I've got an
>>> impression that it may be left uncommitted and thus it may cause data
>>> corruption. I just would like a simple confirmation if code which
>>> commits cache on cut-over to new host does exist and no data corruption
>>> due to RBD cache+live migration should happen.
>>>
>>> Regards,
>>> Vladimir
>>
>> QEMU does a flush on all the disks when it stops the guest on the
>> original host, so there will be no uncommitted data in the cache.
>>
>> Josh
> Thank you for quick and precise answer. Now when I actually attempted to
> live migrate ceph based VM I get:
>
> Unable to migrate guest: Invalid relative path
> 'rbd/mail.logics.net.au:rbd_cache=true': Invalid argument
>
> I guess KVM does not like having :rbd_cache=true (migration works
> without it). I know that it is most likely KVM problem but still decided
> to ask here in case if you know about it. Any ideas how to fix it?
>
> Regards,
> Vladimir

Is the destination librbd older and not supporting the cache option?

Migrating with rbd_cache=true and other options specified like that
worked in my testing.

Josh