From owner-freebsd-scsi@FreeBSD.ORG Fri Oct 27 21:40:24 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 4FB5A16A415 for ; Fri, 27 Oct 2006 21:40:24 +0000 (UTC) (envelope-from cracauer@koef.zs64.net) Received: from koef.zs64.net (koef.zs64.net [212.12.50.230]) by mx1.FreeBSD.org (Postfix) with ESMTP id B3C3343D62 for ; Fri, 27 Oct 2006 21:40:23 +0000 (GMT) (envelope-from cracauer@koef.zs64.net) Received: from koef.zs64.net (koef.zs64.net [212.12.50.230]) by koef.zs64.net (8.13.8/8.13.8) with ESMTP id k9RLeMDx087083 for ; Fri, 27 Oct 2006 23:40:22 +0200 (CEST) (envelope-from cracauer@koef.zs64.net) Received: (from cracauer@localhost) by koef.zs64.net (8.13.8/8.13.8/Submit) id k9RLeMdW087082 for freebsd-scsi@freebsd.org; Fri, 27 Oct 2006 17:40:22 -0400 (EDT) (envelope-from cracauer) Date: Fri, 27 Oct 2006 17:40:22 -0400 From: Martin Cracauer To: freebsd-scsi@freebsd.org Message-ID: <20061027214011.GB86642@cons.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.4.2.2i Subject: 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 21:40:24 -0000 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 -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Martin Cracauer http://www.cons.org/cracauer/ FreeBSD - where you want to go, today. http://www.freebsd.org/