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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Mon, 20 Oct 2008 16:14:42 +0200
User-agent: Thunderbird (X11/20080723)

Jens Axboe wrote:
Use hdparm, it's an ATA drive even if Linux currently uses the scsi
layer for it. Or use sysfs, there's a "cache_type" attribute in the scsi
disk sysfs directory.

With this, I was able to benchmark the write cache for 4k and 128k random access loads. Numbers in iops, hope it doesn't get mangled:

               4k blocks            128k blocks
pattern   cache off   cache on  cache off   cache on
read       103         101         74         71
write       86         149         72         91
rw          87          89         63         65

Test was run on a 90G logical volume of a 250G laptop disk; using O_DIRECT and libaio.

Pure write workloads see a tremendous benefit, likely because the heads can do a linear scan of the disk. An 8MB cache translates to 2000 objects, likely around 1000 per pass. Increasing the block size reduces the performance boost, as expected.

read/write workloads do not benefit at all (or maybe a bit); presumably the head movement is governed by reads alone.

Of course, this tests only the disk subsystem; in particular, if some workload is sensitive to write latencies, the write cache can reduce those in a mixed read/write load, as long as the cache is not flooded (so loads with a lower percentage of writes would benefit more).

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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