From owner-freebsd-hackers@FreeBSD.ORG Wed Jul 25 22:36:24 2012 Return-Path: Delivered-To: freebsd-hackers@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 0E520106564A; Wed, 25 Jul 2012 22:36:24 +0000 (UTC) (envelope-from avg@FreeBSD.org) Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140]) by mx1.freebsd.org (Postfix) with ESMTP id C4C7D8FC15; Wed, 25 Jul 2012 22:36:22 +0000 (UTC) Received: from porto.starpoint.kiev.ua (porto-e.starpoint.kiev.ua [212.40.38.100]) by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id BAA24384; Thu, 26 Jul 2012 01:36:21 +0300 (EEST) (envelope-from avg@FreeBSD.org) Received: from localhost ([127.0.0.1]) by porto.starpoint.kiev.ua with esmtp (Exim 4.34 (FreeBSD)) id 1SuABp-0009Bn-9I; Thu, 26 Jul 2012 01:36:21 +0300 Message-ID: <501074E4.4040805@FreeBSD.org> Date: Thu, 26 Jul 2012 01:36:20 +0300 From: Andriy Gapon User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:13.0) Gecko/20120620 Thunderbird/13.0.1 MIME-Version: 1.0 To: Alexander Motin References: <501056C4.3080806@FreeBSD.org> <50106E5F.4030402@FreeBSD.org> In-Reply-To: <50106E5F.4030402@FreeBSD.org> X-Enigmail-Version: 1.4.2 Content-Type: text/plain; charset=x-viet-vps Content-Transfer-Encoding: 7bit Cc: freebsd-scsi@FreeBSD.org, freebsd-hackers@FreeBSD.org, freebsd-geom@FreeBSD.org Subject: Re: geom <-> cam disk X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 25 Jul 2012 22:36:24 -0000 on 26/07/2012 01:08 Alexander Motin said the following: > Different controllers have different command queueing limitations. If you are > testing with ahci(4) driver and modern disks, then their 32 command slots per > port can be enough for many workloads to enqueue all commands to the hardware > and leave queue empty as you've described. But if you take harder workload, or > controller/ device without command queueing support, extra requests will be > accumulated on that bioq and sorted there. Alexander, thank you for the reply. Indeed, using 64 parallel dd processes with bs=512 I was able to 'kick in' the disksort logic. But I am not sure if the disksort algorithm makes much difference in this case given the number of commands that a disk firmware can internally re-order. (Not mentioning that potentially disksort could starve some I/O bound processes in favor of others -- but that's a totally different topic). But then, of course, for the less capable hardware the disksort could still be a significant factor. -- Andriy Gapon