linux-kernel - Re: Why is NCQ enabled by default by libata? (2.6.20)

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Message-ID: <4608B2B9.7090503@garzik.org>
Date:	Tue, 27 Mar 2007 01:59:21 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
CC:	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>
Subject: Re: Why is NCQ enabled by default by libata? (2.6.20)

Justin Piszcz wrote:
> Without NCQ, performance is MUCH better on almost every operation, with 
> the exception of 2-3 items.

Variables to take into account:

* the drive (NCQ performance wildly varies)
* the IO scheduler
* the filesystem (if not measuring direct to blkdev)
* application workload (or in your case, benchmark tool)
	* in particular, the threaded-ness of the apps

For the overwhelming majority of combinations, NCQ should not /hurt/ 
performance.

For the majority of combinations, NCQ helps (though it may not be often 
that you use more than 4-8 tags).

In some cases, NCQ firmware may be broken.  There is a Maxtor firmware 
id, and some Hitachi ids that people are leaning towards recommending be 
added to the libata 'horkage' list.

	Jeff

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