From owner-freebsd-hackers Tue Mar 3 00:49:15 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id AAA01275 for freebsd-hackers-outgoing; Tue, 3 Mar 1998 00:49:15 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from allegro.lemis.com (allegro.lemis.com [192.109.197.134]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id AAA01265 for ; Tue, 3 Mar 1998 00:49:10 -0800 (PST) (envelope-from grog@lemis.com) Received: from freebie.lemis.com (freebie.lemis.com [192.109.197.137]) by allegro.lemis.com (8.8.7/8.8.5) with ESMTP id TAA09407; Tue, 3 Mar 1998 19:19:07 +1030 (CST) Received: (from grog@localhost) by freebie.lemis.com (8.8.8/8.8.7) id TAA16738; Tue, 3 Mar 1998 19:19:06 +1030 (CST) (envelope-from grog) Message-ID: <19980303191906.19116@freebie.lemis.com> Date: Tue, 3 Mar 1998 19:19:06 +1030 From: Greg Lehey To: shimon@simon-shapiro.org Cc: hackers@FreeBSD.ORG, blkirk@float.eli.net, jdn@acp.qiv.com, wilko@yedi.iaf.nl, tlambert@primenet.com, sbabkin@dcn.att.com Subject: Re: SCSI Bus redundancy... References: <19980303084608.56831@freebie.lemis.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.89i In-Reply-To: ; from Simon Shapiro on Mon, Mar 02, 1998 at 05:48:19PM -0800 WWW-Home-Page: http://www.lemis.com/~grog Organization: LEMIS, PO Box 460, Echunga SA 5153, Australia Phone: +61-8-8388-8286 Fax: +61-8-8388-8725 Mobile: +61-41-739-7062 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Mon, 2 March 1998 at 17:48:19 -0800, Simon Shapiro wrote: > > On 02-Mar-98 Greg Lehey wrote: > > ... > >> That's not the point. OK, we were talking about RAID 5 here, which >> also has parity blocks, but the point is that if you add another disk, >> you're effectively adding another block every n blocks in the file >> system address space. It requires some non-trivial data movement to >> rearrange all the data (more specifically, except for the first n (n = >> old number of drives) blocks, you must move *everything*, and you must >> recalculate parity for every stripe. > > Not quite. The [parity is not in the filesystem. It is in the ``device''. > The filesystem sees a plain, old LBA addressable ``disk''. If a RAID-5 > array grows, the ``disk'' will grow by having its last block address be > (old_size - 1) + increment. Yes, of course. Sorry for my sloppy terminology. >> My question ("How can that work?") was based on the misassumption that >> this would be too much work to be justifiable. > > ``Justifiable'' is a relative term. If the cost is 30% reduction in > perfromance vs. shutdown of service for 2 hours, that may be real cheap. > Some of the systems we work on measure downtime in minutes/year, and number > of shutdowns in once/several_years. In that scenario, a customer may find > this ability, as complex as it may be, quite attractive. Agreed. Greg To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message