Date: Mon, 2 Mar 1998 23:57:44 +0100 (MET) From: Wilko Bulte <wilko@yedi.iaf.nl> To: grog@lemis.com (Greg Lehey) Cc: sbabkin@dcn.att.com, tlambert@primenet.com, shimon@simon-shapiro.org, jdn@acp.qiv.com, blkirk@float.eli.net, hackers@FreeBSD.ORG Subject: Re: SCSI Bus redundancy... Message-ID: <199803022257.XAA06604@yedi.iaf.nl> In-Reply-To: <19980303084608.56831@freebie.lemis.com> from Greg Lehey at "Mar 3, 98 08:46:08 am"
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As Greg Lehey wrote... > 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. And apart from the work involved to get it implemented: how long would it take a RAIDset to get re-organised/enlarged. Reason #1 for doing things like this is because you don't want downtime. And I don't want to think about some hardware failure (say a disk) halfway during this process. That would really result in a dis[k]array ;-) 'Not everything that can be done should be done' Wilko _ ______________________________________________________________________ | / o / / _ Bulte email: wilko @ yedi.iaf.nl http://www.tcja.nl/~wilko |/|/ / / /( (_) Arnhem, The Netherlands - Do, or do not. There is no 'try' --------------- Support your local daemons: run [Free,Net,Open]BSD Unix -- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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