[conspire] DMA for SATA? -was: SATA HD transfers slow?

[Mark S Bilk]

On Sat, Jul 24, 2010 at 07:02:27PM -0400, Luke S Crawford wrote:
>...

Luke, many thanks for your very informative answers to my two
previous posts!

But every SATA transfer is still using up one of my two CPU cores.
See the picture of the KDE4 System Monitor, downloadable from here:  
http://cosmicpenguin.com/tmp/SysMon.jpg

The wide peak is the transfer, and the narrow ones about every 
second are Firefox.

So I think SATA is using PIO.  How can I enable DMA?  hdparm fails:

hdparm -d /dev/sdb

/dev/sdb:
 HDIO_GET_DMA failed: Inappropriate ioctl for device

Here are copies of the CPU line from "top".  The numbers add up
to 100%, so the measurements for both cores are being added 
together.  The transfer is using 30-45% wa (wait?) and 4 or 8% 
system (for disk-disk or disk-/dev/null).  Added and multiplied 
by 2 these equal about 100% of one of the cores.  Each transfer
is a 700+MB file.  I used a new one each time so it wouldn't be
in disk cache.

idle
Cpu(s): 11.9%us,  3.2%sy,  0.0%ni, 84.8%id,  0.0%wa,  0.0%hi,  0.2%si,  0.0%st
Cpu(s): 11.3%us,  2.7%sy,  0.0%ni, 85.2%id,  0.8%wa,  0.0%hi,  0.0%si,  0.0%st

idle, during one of Firefox's peaks
Cpu(s): 45.4%us,  2.5%sy,  0.0%ni, 51.5%id,  0.7%wa,  0.0%hi,  0.0%si,  0.0%st

SATA to SATA transfer using mc
Cpu(s): 39.5%us, 15.0%sy,  0.0%ni, 13.9%id, 29.7%wa,  0.3%hi,  1.5%si,  0.0%st
Cpu(s): 31.2%us, 14.1%sy,  0.0%ni, 20.4%id, 32.8%wa,  0.2%hi,  1.3%si,  0.0%st

SATA to SATA transfer using cp
Cpu(s): 28.0%us, 10.7%sy,  0.0%ni, 24.6%id, 36.0%wa,  0.2%hi,  0.5%si,  0.0%st
Cpu(s): 32.4%us, 11.7%sy,  0.0%ni, 19.0%id, 35.3%wa,  0.5%hi,  1.1%si,  0.0%st

SATA to /dev/null transfer using cp
Cpu(s): 16.9%us,  9.1%sy,  0.0%ni, 34.7%id, 37.7%wa,  0.5%hi,  1.1%si,  0.0%st

SATA to /dev/null transfer using cp, probably during a Firefox peak
Cpu(s): 49.2%us,  8.3%sy,  0.0%ni,  4.8%id, 34.5%wa,  1.0%hi,  2.3%si,  0.0%st

SATA to /dev/null transfer using cp, with Firefox stopped
Cpu(s):  3.3%us,  7.4%sy,  0.0%ni, 43.6%id, 44.7%wa,  0.3%hi,  0.6%si,  0.0%st