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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 2.0.0.14 (X11/20080501) |
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.
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