From owner-freebsd-hackers Mon Mar 2 18:07:55 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id SAA09555 for freebsd-hackers-outgoing; Mon, 2 Mar 1998 18:07:55 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from sendero.simon-shapiro.org (sendero-fxp0.Simon-Shapiro.ORG [206.190.148.34]) by hub.freebsd.org (8.8.8/8.8.8) with SMTP id SAA09468 for ; Mon, 2 Mar 1998 18:07:34 -0800 (PST) (envelope-from shimon@sendero-fxp0.simon-shapiro.org) Received: (qmail 4996 invoked by uid 1000); 3 Mar 1998 01:48:19 -0000 Message-ID: X-Mailer: XFMail 1.3-alpha-021598 [p0] on FreeBSD X-Priority: 3 (Normal) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit MIME-Version: 1.0 In-Reply-To: <19980303084608.56831@freebie.lemis.com> Date: Mon, 02 Mar 1998 17:48:19 -0800 (PST) Reply-To: shimon@simon-shapiro.org Organization: The Simon Shapiro Foundation From: Simon Shapiro To: Greg Lehey Subject: Re: SCSI Bus redundancy... Cc: hackers@FreeBSD.ORG, blkirk@float.eli.net, jdn@acp.qiv.com, wilko@yedi.iaf.nl, tlambert@primenet.com, sbabkin@dcn.att.com Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG 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. > 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. ---------- Sincerely Yours, Simon Shapiro Shimon@Simon-Shapiro.ORG Voice: 503.799.2313 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message