linux-kernel - Re: Lower HD transfer rate with NCQ enabled?

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Date:	Tue, 03 Apr 2007 12:47:40 -0400
From:	Mark Lord <lkml@....ca>
To:	Chris Snook <csnook@...hat.com>
Cc:	Paa Paa <paapaa125@...mail.com>, linux-kernel@...r.kernel.org
Subject: Re: Lower HD transfer rate with NCQ enabled?

Chris Snook wrote:
> 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. 

No, NCQ was designed to optimize *server* workloads:  lots of *small*,
random I/O's.

But WD drives, in particular the Raptor series, have a firmware "feature"
that disables "drive readahead" whenever NCQ is in use.

So they will perform poorly only any medium/large sequential access
if NCQ is employed.  Which is why the custom MS drivers avoid NCQ when
doing large sequential accesses.  Ours don't do this, yet.

-ml
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