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

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Message-ID: <460C0AB6.9020709@garzik.org>
Date:	Thu, 29 Mar 2007 14:51:34 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Phillip Susi <psusi@....rr.com>
CC:	Justin Piszcz <jpiszcz@...idpixels.com>, linux@...izon.com,
	htejun@...il.com, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Why is NCQ enabled by default by libata? (2.6.20)

Phillip Susi wrote:
> Jeff Garzik wrote:
>> NCQ provides for a more asynchronous flow.  It helps greatly with 
>> reads (of which most are, by nature, synchronous at the app level) 
>> from multiple threads or apps.  It helps with writes, even with write 
>> cache on, by allowing multiple commands to be submitted and/or retired 
>> at the same time.
> 
> But when writing, what is the difference between queuing multiple tagged 
> writes, and sending down multiple untagged cached writes that complete 
> immediately and actually hit the disk later?  Either way the host keeps 
> sending writes to the disk until it's buffers are full, and the disk is 
> constantly trying to commit those buffers to the media in the most 
> optimal order.

Less overhead to starting commands, and all the other benefits of making 
operations fully async.

	Jeff



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