NCQ usage/support in linux

["xerces8" <xerces8@butn.net>]

Hello!

I have a few questions about SATA NCQ.

Background story:
Many users noticed that concurrent (by two programs) read access to a disk
under Windows goes very slow with modern systems, using SATA AHCI mode.

References:
 - "Slower concurrent disk access with NCQ ?"
<http://forums.hexus.net/hexus-hardware/138640-slower-concurrent-disk-access-ncq.html>
 - "Slower with NCQ ?, Concurent access" <http://forums.storagereview.net/index.php?showtopic=26864>
 - "NCQ: Best Upgrade For a Power User!" <http://forums.storagereview.net/index.php?showtopic=26965>

(for short summary you can read the 3rd page of the last forum topic linked above)

Here is the gist of the issue:
--quote from a forum post--
320GB WD Caviar:

Windows:
63 MB/s: 1 instance
28 MB/s: 2 instances (both at 0% position)
9 MB/s: 2 instances (0% position and 90% position)
15 MB/s: 10 instances (10% gap between each)

Linux:
63 MB/s: 1 instance
63 MB/s: 2 instances (both at 0% position)
45 MB/s: 2 instances (0% position and 90% position)
48 MB/s: 10 instances (10% gap between each)

Linux, stock install of Fedora 9 x64 (2.4.25 kernel).

Linux commands issued:
dd if=/dev/sdb of=/dev/null bs=256K skip=200K (skip was incremented by 200K each instance which
corresponds to about 50GB forward in this 320GB drive)

all dd invocations started at the same time (batch).

throughput measured with:
iostat -m 1 /dev/sdb (m shows in megabytes, 1 is the update interval of graph every second)

Windows commands:
dd if=\\.\PHYSICALDRIVE1 of=NUL bs=256K skip=200K (as above)
--end quote--

My question is: How does linux deal with NCQ ?
Is it used by default ? Any known problems with it ?

In the forum the Seagate 7200.11 harddrive family is mentioned as the only one
not having the performance issue. Does those drives also behave exceptionally
in linux ?

Thanks for any answers.
Sincerely,
David