From owner-freebsd-hackers Mon Mar 2 14:17:28 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id OAA29190 for freebsd-hackers-outgoing; Mon, 2 Mar 1998 14:17:28 -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 OAA29135 for ; Mon, 2 Mar 1998 14:17:19 -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 IAA08840; Tue, 3 Mar 1998 08:46:16 +1030 (CST) Received: (from grog@localhost) by freebie.lemis.com (8.8.8/8.8.7) id IAA13247; Tue, 3 Mar 1998 08:46:09 +1030 (CST) (envelope-from grog) Message-ID: <19980303084608.56831@freebie.lemis.com> Date: Tue, 3 Mar 1998 08:46:08 +1030 From: Greg Lehey To: sbabkin@dcn.att.com, tlambert@primenet.com Cc: shimon@simon-shapiro.org, wilko@yedi.iaf.nl, jdn@acp.qiv.com, blkirk@float.eli.net, hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.89i In-Reply-To: ; from sbabkin@dcn.att.com on Mon, Mar 02, 1998 at 02:23:50PM -0500 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 14:23:50 -0500, sbabkin@dcn.att.com wrote: >> ---------- >> From: Terry Lambert[SMTP:tlambert@primenet.com] >> >>>>> I think Julian's SLICE code has something in that direction. >> DPT >>>>> supports INCREASING the size of a RAID-5 array by adding drives. >>>> >>>> How can that work? >>> >>> Something like >>> - read N RAID blocks from K disks >>> - compute new checksum for K+1 disks and write as less number >>> of RAID blocks but each one of bigger size (K+1/K times) >>> - add empty blocks at the end of RAID in the added space >> >> You would have to remember to grab the blocks to be relocated with >> the same O(n) randomness as their allocation. 8-). >> > Huh ? Probably I've missed something about RAIDs. I've thought > that, for example, RAID block 0 consists of blocks 0 of all > the physical disks. And so on. And I've thought that RAID itself > does not allocate any blocks, the upper level like filesystem or > volume manager does it, RAID just makes chechsuming. Am I wrong again ? 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. My question ("How can that work?") was based on the misassumption that this would be too much work to be justifiable. Greg To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message