Date: Mon, 02 Mar 1998 17:48:19 -0800 (PST) From: Simon Shapiro <shimon@simon-shapiro.org> To: Greg Lehey <grog@lemis.com> 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... Message-ID: <XFMail.980302174819.shimon@simon-shapiro.org> In-Reply-To: <19980303084608.56831@freebie.lemis.com>
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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
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