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Date: Mon, 23 Jun 2008 05:50:03 +0300
From: Avi Kivity <avi@qumranet.com>
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Subject: Re: [Qemu-devel] [PATCH] qcow2: improve I/O performance with cache=off
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To: qemu-devel@nongnu.org

Laurent Vivier wrote:
> Hi,
>
> this patch improves qcow2 I/O performance when used with cache=off.
>
> It modifies qcow_aio_[read|write]_cb() to read as many clusters as
> possible per bdrv_aio_[read|write]().
>
>   

Write allocation is still slow though.  For fixing this, I think the 
only way is to modify get_cluster_offset() to accept and return an extent.

> I've made some tests with dbench:
>
> 				WITHOUT PATCH	WITH PATCH
>
> ide, cache=off,snapshot=off     20.8494 MB/sec  24.0711 MB/sec
> ide, cache=off,snapshot=on      20.9349 MB/sec  24.5031 MB/sec
> scsi,cache=off,snapshot=off     21.0264 MB/sec  24.6119 MB/sec
> scsi,cache=off,snapshot=on      21.4184 MB/sec  24.6739 MB/sec
>
> The gain is approximately 15%.
>   

I don't think dbench is a good test for this as it involves the guest 
pagecache.  'dd' or 'fio' would be much more indicative.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.