Re: how to debug slow rbd block device

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

On 05/23/2012 02:03 AM, Andrey Korolyov wrote:
> Hi Josh,
>
> Can you please answer to list on this question? It is important when
> someone wants to build HA KVM cluster on the rbd backend and needs to
> wc cache. Thanks!
>
> On Wed, May 23, 2012 at 10:30 AM, Josh Durgin<josh.durgin@inktank.com>  wrote:
>> On 05/22/2012 11:18 PM, Stefan Priebe - Profihost AG wrote:
>>>
>>> Hi,
>>>
>>>>> So try enabling RBD writeback caching — see http://marc.info
>>>>> /?l=ceph-devel&m=133758599712768&w=2
>>>>> will test tomorrow. Thanks.
>>>
>>> Can we path this to the qemu-drive option?
>>
>>
>> Yup, see http://article.gmane.org/gmane.comp.file-systems.ceph.devel/6400
>>
>> The normal qemu cache=writeback/writethrough/none option will work in qemu
>> 1.2.
>>
>> Josh
>
> By the way, is it possible to flush cache outside? I may need that for
> VM live migration and such hook will be helpful.

Qemu will do that for you in many cases, but it looks like we need to 
implement bdrv_invalidate_cache to make live migration work.

http://tracker.newdream.net/issues/2467

librbd itself flushes the cache when a snapshot is created or the image 
is closed, but there's no way to trigger it directly right now.

http://tracker.newdream.net/issues/2468
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