Subject: Re: disk speed
To: Wolfgang Rupprecht <wolfgang+gnus20020522T061435@wsrcc.com>
From: Manuel Bouyer <bouyer@antioche.eu.org>
List: netbsd-users
Date: 05/22/2002 21:31:43
On Wed, May 22, 2002 at 06:28:18AM -0700, Wolfgang Rupprecht wrote:
>
> I've just noticed a strange performance anomaly in the disk transfer
> speed (under -current/x86 w. Promise ATA-100 and Seagate Barracuda
> IV's, with wd1 being an IDE "slave" of wd0).
>
> If I set up a dd transfer from a raw disk partition to /dev/null I get
> ~40Mbytes/sec. In theory ATA-100 can support 2 of these xfers going
> full bore, and I was expecting to see the transfer speed drop a bit.
> What I wasn't expecting was to see the disk xfer drop in less than
> half to ~13MBytes/sec.
>
> Transfer from wd0 only:
>
> wd0 wd1 cpu
> KB/t t/s MB/s KB/t t/s MB/s us ni sy in id
>
> 64.00 640 39.98 0.00 0 0.00 2 0 5 0 93
> 63.22 643 39.67 0.00 0 0.00 0 0 1 0 99
> 64.00 639 39.91 0.00 0 0.00 3 0 2 0 95
> 64.00 639 39.91 0.00 0 0.00 0 0 1 0 99
>
> Transfer from wd1 only:
>
> wd0 wd1 cpu
> KB/t t/s MB/s KB/t t/s MB/s us ni sy in id
>
> 0.00 0 0.00 64.00 640 39.98 1 0 0 0 99
> 0.00 0 0.00 64.00 635 39.67 2 0 7 0 91
> 0.00 0 0.00 64.00 639 39.91 1 0 6 0 93
> 8.00 2 0.00 64.00 640 39.98 4 1 2 0 93
>
> Transfer from both wd0 and wd1:
>
> wd0 wd1 cpu
> KB/t t/s MB/s KB/t t/s MB/s us ni sy in id
>
> 64.00 200 12.50 64.00 209 13.0 2 1 0 0 97
> 64.00 210 13.12 64.00 211 13.1 0 0 2 0 98
> 64.00 222 13.86 64.00 221 13.8 0 0 0 0 100
> 63.69 177 11.02 64.00 270 16.8 3 0 1 0 96
>
> Anyone have a good model for what is going on here?
IDE doesn't do disconnect/reselect, so when you have 2 concurent one drive
is idle while the other one is working. So in your test it's expected
to have the per-drive transfer rate drop by half.
I'm not sure why it's more than half. Maybe the extra delay between 2 transfers
prevents the drive from doing some kind of "streaming" ?
--
Manuel Bouyer <bouyer@antioche.eu.org>
--