Re: Lower HD transfer rate with NCQ enabled?

[Chris Snook <csnook@redhat.com>]

Paa Paa wrote:
> 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?

hdparm -t is a perfect example of a synthetic benchmark.  NCQ was 
designed to optimize real-world workloads.  The overhead gets hidden 
pretty well when there are multiple requests in flight simultaneously, 
as tends to be the case when you have a user thread reading data while a 
kernel thread is asynchronously flushing the user thread's buffered 
writes.  Given that you're breaking even with one user thread and one 
kernel thread doing I/O, you'll probably get performance improvements 
with higher thread counts.

	-- Chris