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[Qemu-devel] Re: [RFC][PATCH 2/2] qcow2: Use bdrv_(p)write_sync for meta

From: Kevin Wolf
Subject: [Qemu-devel] Re: [RFC][PATCH 2/2] qcow2: Use bdrv_(p)write_sync for metadata writes
Date: Thu, 17 Jun 2010 16:39:54 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

Am 17.06.2010 16:19, schrieb Stefan Hajnoczi:
> On Thu, Jun 17, 2010 at 1:03 PM, Kevin Wolf <address@hidden> wrote:
>> Use bdrv_(p)write_sync to ensure metadata integrity in case of a crash.
> Any performance numbers?  This change is necessary for correctness but
> I wonder what the performance impact is for users.

No numbers yet, but as you say we need to do it anyway. It should
definitely be better than any other option that I can think of
(cache=writethrough or some O_DIRECT|O_DSYNC mode) in that it only hurts
performance when metadata is actually changed. As long as we only write
guest data, there is no difference.

Making it a barrier instead of a flush would probably be better, have
you already had a look at this since we talked about it?


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