From owner-freebsd-scsi@FreeBSD.ORG Fri Oct 27 21:56:49 2006 Return-Path: X-Original-To: freebsd-scsi@freebsd.org Delivered-To: freebsd-scsi@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 815B516A415 for ; Fri, 27 Oct 2006 21:56:49 +0000 (UTC) (envelope-from scottl@samsco.org) Received: from pooker.samsco.org (pooker.samsco.org [168.103.85.57]) by mx1.FreeBSD.org (Postfix) with ESMTP id 14B6B43D7C for ; Fri, 27 Oct 2006 21:56:48 +0000 (GMT) (envelope-from scottl@samsco.org) Received: from [10.10.3.185] ([165.236.175.187]) (authenticated bits=0) by pooker.samsco.org (8.13.4/8.13.4) with ESMTP id k9RLucLR012836; Fri, 27 Oct 2006 15:56:44 -0600 (MDT) (envelope-from scottl@samsco.org) Message-ID: <45428091.9030208@samsco.org> Date: Fri, 27 Oct 2006 15:56:33 -0600 From: Scott Long User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.7.12) Gecko/20060206 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Martin Cracauer References: <20061027214011.GB86642@cons.org> In-Reply-To: <20061027214011.GB86642@cons.org> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=0.0 required=3.8 tests=none autolearn=failed version=3.1.1 X-Spam-Checker-Version: SpamAssassin 3.1.1 (2006-03-10) on pooker.samsco.org Cc: freebsd-scsi@freebsd.org Subject: Re: Why is SCSI so much faster with the write cache off (than ATA)? X-BeenThere: freebsd-scsi@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: SCSI subsystem List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 27 Oct 2006 21:56:49 -0000 Martin Cracauer wrote: > I have observed it several times and I am missing one bit of knowledge > here: > > why is SCSI so much faster when you turn off the write cache than > P-ATA and SATA? > > P-ATA and SATA crumble to about 1/10th of the speed (just writing one > file with 8k blocks linear), whereas SCSI just loses 10-20%, for me. > I have observed that 10 years ago with some 8 GB ATA IBM disk on the > BX chipset versus some 4 GB Quantum Atlas, and now I see it again with > Seagate 7200.7s and .8s versus a 10K Compaq labled 36 GB drive. > > Personally I don't see why a linear write should be slow at all. > Surely the computer delivers the data fast enough for sectors to be > filled as they pass under the head. Maybe the ATA disks lose one > rotation per sector or per filesystem block written anyway? Then why > don't SCSI disks lose the same way given they are not allowed to cache > anything either? > > Martin The answer: Tagged Queueing The drive can collect a bunch of I/O requests, sort them for optimal layout, then stream their data in all at once. This is not the same thing as buffering through the write cache, but it has a similar performance benefit. Scott