Date: Fri, 12 Dec 1997 13:55:16 -0800 (PST) From: Tom <tom@sdf.com> To: Wilko Bulte <wilko@yedi.iaf.nl> Cc: walcaraz@indy3.gstone.com, freebsd-scsi@freebsd.org Subject: Re: RAID on FreeBSD Message-ID: <Pine.BSF.3.95q.971212134755.22241A-100000@misery.sdf.com> In-Reply-To: <199712121752.SAA01171@yedi.iaf.nl>
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On Fri, 12 Dec 1997, Wilko Bulte wrote: > As Tom wrote... > > > On Thu, 11 Dec 1997, Wilko Bulte wrote: > > > > > > 9 drives in an uncomfortable number for RAID5. Probably better to go > > > > > > Why would 9 drives be uncomfartable? > > > > Well, if you are going to making one arrray of 9 drives, write > > performance will bad. If you are going to making 3 arrays of 3 > > Why? Calculating the parity takes the same overhead in both cases. To do a write, you will hae to read the data back from the other drives to calculate the parity. The more drives, the more data you have to read back, and more i/o you have to do to complete a write. You want to make a tradeoff between parity storage overhead and write performance. > What you *don't* want to do is put too many drives of the same raidset on > a single SCSI bus. Preferably you have only one drive of each set on > each channel. This allows a channel to die completely without loosing your > data. The question was about a 9 drive setup, that presumably going to be put into a single RAID5 array. How would you arrange 9 drives? > > drives each, you will end up with a lot of overhead. > > > > RAID5 arrays of 5 drives is kinda of nice sweet spot. If you go much > > bigger, just use RAID0 over multiple RAID5. > > I don't agree. It *really* depends on the hardware you're using. E.g > the company I work for (DEC) sells the HSZx0 range of controllers. > This controller has (along with battery backup writeback cache) 6 SCSI > device buses. The 'natural' number for that one is 6 drives. Yep, I knew you work DEC, and I knew you were probably baiting me. 5 is closer to 6, than to 9. So I guess you are closer to agreeing with me, than with a 9 drive RAID5 array. I guess the HSZx0 series, is a SCSI-to-SCSI system? Of course that imposes certain limitations as well. > Generally my point is that you really have to take a very close look > to your hardware setup. > > 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]BSD Unix ------ > > Tom
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