[sheepdog] Qemu Live Migration and Cache

[Liu Yuan]

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

Thanks,
Yuan