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Re: [Qemu-devel] [PATCH] qcow2: improve I/O performance with cache=off

From: Avi Kivity
Subject: Re: [Qemu-devel] [PATCH] qcow2: improve I/O performance with cache=off
Date: Mon, 23 Jun 2008 05:50:03 +0300
User-agent: Thunderbird (X11/20080501)

Laurent Vivier wrote:

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 

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