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Re: [Qemu-devel] Re: KVM call minutes for Sept 14

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: KVM call minutes for Sept 14
Date: Wed, 15 Sep 2010 08:52:51 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100826 Lightning/1.0b1 Thunderbird/3.0.7

On 09/15/2010 08:30 AM, Kevin Wolf wrote:
Am 15.09.2010 15:21, schrieb Anthony Liguori:
On 09/15/2010 07:38 AM, Kevin Wolf wrote:
No, we don't really care if the L2 entry is on disk. If the guest want
to have its data safe it needs to issue an explicit flush anyway. The
only thing we want to achieve with bdrv_write_sync is to maintain the
right order between metadata updates to survive a crash without corruption.

Ah, yes, this is brand new :-)

I was looking at my QED branch which is a few weeks old.
Well, the whole bdrv_pwrite_sync thing is new - with your benchmarking
you probably caught qcow2 at its worst performance in years.

FWIW, we queued a run reverting the sync() stuff entirely as we were aware of that. Should have results this morning.

  Initially I
just blindly converted everything to be on the safe side, and now we
need to optimize to get the performance back. There are probably some
more syncs that can be removed in less common paths.

Most likely.


Anthony Liguori


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