[sheepdog] Qemu Live Migration and Cache

[Yibin Shen]

On Thu, Jun 7, 2012 at 5:38 PM, Liu Yuan <namei.unix at gmail.com> wrote:
> On 06/07/2012 05:19 PM, Bastian Scholz wrote:
>
>> I have a question about migration a Qemu Virtual Machines from
>> one node to another...
>>
>> Is there something I have to pay attention when using the
>> farm cache mechanisms?
>
>>
>
>
> Farm doesn't cache any data for regular IO. Instead, Farm caches
> snapshot objects for recovery, which is totally irreverent to data
> consistency of running VM.
>
> Maybe you refer to object cache?
>
>> For example, is it possible, that I ran my virtual Machine V
>> on host A, Start a live migration to host B, but some content
>> in the cache from host A would not known to the rest of the
>> cluster in time?
>>
>
>
> Without object cache, the answer is NO, the data will be known to the
> cluster as soon as the request is completely.
>
> With object cache enabled, probably 'nope' again, but I am not 100%
> sure, live migration mechanism from QEMU will issue a flush request to
> sheep and block for it, then will migrate memory to other machine. For
> the last try with live migration, it did the right thing for me, but
> you'd better test it with your own. When I have time, I'll try it again
> and give you a solid answer.so
qemu will call sync operation before live migration,(nearly) all dirty
objects will be
flushed to sheepdog cluster.

but, if you try 'manual migration', object cache will be quite a problem
>
> Thanks,
> Yuan
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