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?

-wolfgang
-- 
       Wolfgang Rupprecht <wolfgang+gnus@dailyplanet.wsrcc.com>
		    http://www.wsrcc.com/wolfgang/
Coming soon: GPS mapping tools for Open Systems. http://www.gnomad-mapping.com/