Inconsistent IO performance

[Roland Smith]

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.
<snip>
>   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).

Roland
-- 
R.F.Smith                                   http://www.xs4all.nl/~rsmith/
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