Hans Kristian Rosbach [off-list ref] wrote:

-It selects the disk that is closest to the wanted sector by remembering
 what sector was last requested and what disk was used for it.
-For sequential reads (sucha as hdparm) it will override and use the
 same disk anyways. (sector = lastsector+1)

I gained a lot of throughput by alternating disk, but seek time was
roughly doubled. I also tried to get smart and played some with the
code in order to avoid seeking both disks back and forth wildly when
there were two sequential reads. I didn't find a good way to do it
unfortunately.

Interesting. How did you measure latency? Do you have a script you
could post?

I'm not going to make any patch available, because I removed bad-disk
checking in order to simplify it.

The FR1 patch measures disk latency and weights the disk head distances
by the measured latency, which may help.  It probably also gets rid of
that sequential read thing (I haven't done anything but port the patch
to 2.6, not actually run it in anger!).

  ftp://oboe.it.uc3m.es/pub/Programs/fr1-2.15b.tgz

(I am doing a 2.16 with the robust-read patch I suggested added in).

I really don't think this measuring disk head position can help unless
raid controls ALL of the disks in question, or the disks are otherwise
inactive.  Is that your case?


Peter