Re: [Qemu-devel] Storage requirements for live migration

[Mark Wu <wudxw@linux.vnet.ibm.com>]

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On 11/11/2011 08:11 AM, Anthony Liguori wrote:
> I did a brain dump of my understanding of the various storage 
> requirements for live migration.  I think it's accurate but I may have 
> misunderstand some details so I would appreciate review.
>
> I think given sections (1) and (2), the only viable thing is to 
> require cache=none unless we get new interfaces to flush caches.
I have two questions:
1. why is "cache=writethrough" not reliable?  "O_DSYNC" can guarantee 
data is written on disk before the operation is ultimately completed. 
Even if there's synchronization from memory to disk still on-going when 
migration happens,  we needn't care about the dirty pages in host memory 
either because the queued request will be re-executed on destination 
host.  So I think it's reliable too.  Do I miss anything?

2. Can we flush cache for "cache=writeback" in the last phase of 
migration? I think it can resolve the problem of "cache coherent". But 
it could cause an uncertain guest downtime.

Thanks.
Mark

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