linux-kernel - Re: [PATCH] cfq-iosched: NCQ SSDs do not need read queue merging

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Message-ID: <4B4B0AA9.5020900@garzik.org>
Date:	Mon, 11 Jan 2010 06:25:29 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Corrado Zoccolo <czoccolo@...il.com>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>
Subject: Re: [PATCH] cfq-iosched: NCQ SSDs do not need read queue merging

On 01/10/2010 04:04 PM, Corrado Zoccolo wrote:
> NCQ SSDs' performances are not affected by
> distance of read requests, so there is no point in having
> overhead to merge such queues.
>
> Non-NCQ SSDs showed regression in some special cases, so
> they are ruled out by this patch.
>
> This patch intentionally doesn't affect writes, so
> it changes the queued[] field, to be indexed by
> READ/WRITE instead of SYNC/ASYNC, and only compute proximity
> for queues with WRITE requests.
>
> Signed-off-by: Corrado Zoccolo<czoccolo@...il.com>

That's not really true.  Overhead always increases as the total number 
of ATA commands issued increases.

	Jeff




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