From owner-freebsd-alpha Mon Oct 28 18: 9:50 2002 Delivered-To: freebsd-alpha@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 64BBF37B401 for ; Mon, 28 Oct 2002 18:09:49 -0800 (PST) Received: from alcanet.com.au (mail2.alcanet.com.au [203.62.196.17]) by mx1.FreeBSD.org (Postfix) with ESMTP id E8E7743E88 for ; Mon, 28 Oct 2002 18:09:47 -0800 (PST) (envelope-from peter.jeremy@alcatel.com.au) Received: from sydsmtp01.alcatel.com.au (IDENT:root@localhost.localdomain [127.0.0.1]) by alcanet.com.au (8.12.4/8.12.4/Alcanet1.3) with ESMTP id g9T29hC5011609; Tue, 29 Oct 2002 13:09:44 +1100 Received: from gsmx07.alcatel.com.au ([139.188.20.247]) by sydsmtp01.alcatel.com.au (Lotus Domino Release 5.0.11) with ESMTP id 2002102913094409:42218 ; Tue, 29 Oct 2002 13:09:44 +1100 Received: from gsmx07.alcatel.com.au (localhost [127.0.0.1]) by gsmx07.alcatel.com.au (8.12.5/8.12.5) with ESMTP id g9T29hRL060872; Tue, 29 Oct 2002 13:09:43 +1100 (EST) (envelope-from peter.jeremy@alcatel.com.au) Received: (from jeremyp@localhost) by gsmx07.alcatel.com.au (8.12.5/8.12.5/Submit) id g9T29h2Q060871; Tue, 29 Oct 2002 13:09:43 +1100 (EST) (envelope-from peter.jeremy@alcatel.com.au) Date: Tue, 29 Oct 2002 13:09:43 +1100 From: Peter Jeremy To: "Wilkinson,Alex" Cc: alpha@FreeBSD.ORG Subject: Re: [hardware] Tagged Command Queuing or Larger Cache ? Message-ID: <20021029020943.GI6446@gsmx07.alcatel.com.au> Mail-Followup-To: "Wilkinson,Alex" , alpha@FreeBSD.ORG References: <20021029095516.G91719-100000@squirm.dsto.defence.gov.au> Mime-Version: 1.0 In-Reply-To: <20021029095516.G91719-100000@squirm.dsto.defence.gov.au> User-Agent: Mutt/1.4i X-MIMETrack: Itemize by SMTP Server on SYDSMTP01/AlcatelAustralia(Release 5.0.11 |July 24, 2002) at 29/10/2002 01:09:44 PM, Serialize by Router on SYDSMTP01/AlcatelAustralia(Release 5.0.11 |July 24, 2002) at 29/10/2002 01:09:45 PM, Serialize complete at 29/10/2002 01:09:45 PM Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: owner-freebsd-alpha@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.org On 2002-Oct-29 09:55:49 +1030, "Wilkinson,Alex" wrote: >Do the benifits from a having a larger disk cache such as the "WD >40GB 7200RPM w/8MB Cache" has, outweigh the benefits of Tagged >Command Queuing ? This is somewhat off-topic for -alpha... The quick answer is: "Not if you value your data". The longer answer is: Without tagged queueing, the device driver cannot determine when the data for a particular write command has been committed to the media (as against just being stored in the cache). Soft-updates (in particular) relies on accurate write ordering to ensure that the filesystem on disk is always consistent. Having a volatile cache in the disk breaks this assumption - if you have a power failure, you don't know what was lost. The safe states are: tagging & caching or no-caching (which is slow). Peter To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-alpha" in the body of the message