linux-kernel - Re: SATA RAID5 speed drop of 100 MB/s

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Message-ID: <467CC5C5.6040201@garzik.org>
Date:	Sat, 23 Jun 2007 03:03:33 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Carlo Wood <carlo@...noe.com>
CC:	Tejun Heo <htejun@...il.com>, Manoj Kasichainula <manoj@...com>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>
Subject: Re: SATA RAID5 speed drop of 100 MB/s

Carlo Wood wrote:
> The dmesg output of 33480a0ede8dcc7e6483054279008f972bd56fd3 (thus
> "before") is:
[...]
> And the dmesg output of 551c012d7eea3dc5ec063c7ff9c718d39e77634f (thus
> "after") is:
[...]

Your disk configurations are quite radically different between the two 
kernels (see attached diff for key highlights).

The new behavior of the more recent kernel (551c012d7...) is that it now 
fully drives your hardware :)  The reset problems go away, NCQ is 
enabled, and if you had 3.0Gbps drives (you don't) they would be driven 
at a faster speed.

Given that some drives might be better tuned for benchmarks in 
non-queued mode, and that a major behavior difference is that your 
drives are now NCQ-enabled, the first thing I would suggest you try is 
disabling NCQ:
	http://linux-ata.org/faq.html#ncq

Other indicators are the other changes in the "ahci 0000:00:1f.2: 
flags:" line, which do affect other behaviors, though none so important 
to RAID5 performance as NCQ, I would think.

Turning on NCQ also potentially affects barrier behavior in RAID, though 
I'm guessing that is not a factor here.

	Jeff

View attachment "diff.txt" of type "text/plain" (1674 bytes)