Inconsistent IO performance
rsmith at xs4all.nl
Sat Aug 14 22:07:43 UTC 2010
On Sat, Aug 14, 2010 at 02:36:31AM +0200, Roland Smith wrote:
> On Fri, Aug 13, 2010 at 01:36:12PM -1000, Clifton Royston wrote:
> > > Both figures seem quite low to me? I cannot exactly reproduce your test,
> > > because I don't have an empty second disk handy, but doing
> > >
> > > dd if=/dev/zero bs=1m count=100 of=/tmp/foo
> > With a total write size of 100MB, aren't you just testing speed of
> > writing into cache RAM this way? I think you need to write amounts
> > dramatically greater than the total size of the RAM to get values which
> > appropriately measure disk speed.
> > This also supports that theory - off the top of my head, maximum
> > theoretical possible write throughput to a similarly sized 7200rpm
> > drive should be 70MB/s (buffer to disk data transfer rate according to
> > WDC's specs.) <http://wdc.com/en/library/sata/2879-701277.pdf>
> Ok, so I tried this;
> dd if=/dev/zero of=/tmp/foo bs=10M count=1000
> 10485760000 bytes transferred in 138.304953 secs (75816229 bytes/sec)
> 10485760000 bytes transferred in 139.125501 secs (75369073 bytes/sec)
> 10485760000 bytes transferred in 136.149871 secs (77016305 bytes/sec)
> Which is around 72 MiB/s with filesystem overhead, which sounds about
> right. The drive was making plenty of noise. The point is that it is _way_
> more than the 18-22 MiB/s on a raw disk that Kevin is getting.
> I'll try the same on my laptop topmorrow and see what that gets me. This desktop
> machine is ICH7 with ata(4), laptop is ICH9 with ahci(4).
Figures from the laptop running 8.1-RELEASE amd64, ahci driver with the
ada0: <ST9320423AS 0002SDM1> ATA-8 SATA 2.x device
ada0: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada0: Command Queueing enabled
ada0: 305245MB (625142448 512 byte sectors: 16H 63S/T 16383C)
Running the same test;
dd if=/dev/zero of=/tmp/foo bs=10M count=1000
Gives the following results.
10485760000 bytes transferred in 122.625997 secs (85510090 bytes/sec)
10485760000 bytes transferred in 126.081170 secs (83166741 bytes/sec)
10485760000 bytes transferred in 126.101845 secs (83153105 bytes/sec)
Which is about 10% faster than on the desktop.
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