Lower HD transfer rate with NCQ enabled?

["Paa Paa" <paapaa125@hotmail.com>]

I'm using Linux 2.6.20.4. I noticed that I get lower SATA hard drive 
throughput with 2.6.20.4 than with 2.6.19. The reason was that 2.6.20 
enables NCQ by defauly (queue_depth = 31/32 instead of 0/32). Transfer rate 
was measured using "hdparm -t":

With NCQ (queue_depth == 31): 50MB/s.
Without NCQ (queue_depth == 0): 60MB/s.

20% difference is quite a lot. This is with Intel ICH8R controller and 
Western Digital WD1600YS hard disk in AHCI mode. I also used the next 
command to cat-copy a biggish (540MB) file and time it:

rm temp && sync && time sh -c 'cat quite_big_file > temp && sync'

Here I noticed no differences at all with and without NCQ. The times (real 
time) were basically the same in many successive runs. Around 19s.

Q: What conclusion can I make on "hdparm -t" results or can I make any 
conclusions? Do I really have lower performance with NCQ or not? If I do, is 
this because of my HD or because of kernel?

Best regards,
Paapaa

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