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

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Message-ID: <467DA1F5.2060306@garzik.org>
Date:	Sat, 23 Jun 2007 18:43:01 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Carlo Wood <carlo@...noe.com>, Jeff Garzik <jeff@...zik.org>,
	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:
> Is it possible that the measurement with "hdparm -tT" returns a higher
> value for some setting, but that the over-all real-life performance
> drops?

IN THEORY, RAID performance should /increase/ due to additional queued 
commands available to be sent to the drive.  NCQ == command queueing == 
sending multiple commands to the drive, rather than one-at-a-time like 
normal.

But hdparm isn't the best test for that theory, since it does not 
simulate the transactions like real-world MD device usage does.

We have seen buggy NCQ firmwares where performance decreases, so it is 
possible that NCQ just isn't good on your drives.

	Jeff

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