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

[Kevin Wolf]

Am 11.11.2011 07:27, schrieb Mark Wu:
> 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?

The problem is about reading. The destination host reads some pieces of
information during its initialisation, which may later be changed on the
host. After the migration has completed, the destination must not use
the old data that it read previously and that might be in its cache, but
it must access the storage. O_DSYNC doesn't help with that, as reading
still uses the cache there.

Kevin