Date: Fri, 5 Mar 2004 13:18:00 -0500 From: Charles Swiger <cswiger@mac.com> To: Jan Grant <Jan.Grant@bristol.ac.uk> Cc: freebsd-questions@freebsd.org Subject: Re: 1 processor vs. 2 Message-ID: <6F5934EE-6ED1-11D8-BB2A-003065ABFD92@mac.com> In-Reply-To: <Pine.GSO.4.58.0403051053560.22978@mail.ilrt.bris.ac.uk> References: <BC6A0533.1DC4C%joe@jwebmedia.com> <20040303222612.W39053@guldivar.globalwire.se> <20040303213641.GA37555@happy-idiot-talk.infracaninophile.co.uk> <200403040050.53556.danny@ricin.com> <40474997.60403@mac.com> <Pine.GSO.4.58.0403051053560.22978@mail.ilrt.bris.ac.uk>
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On Mar 5, 2004, at 5:57 AM, Jan Grant wrote: > How did you come to this conclusion? For a RAID 5 with a single parity > drive, the reason you zero the disks out completely on initialisation > is > to set up the integrity of the parity check. Then any update to any > RAID5 with single parity requires a read of two drives (the target > sector and the corresponding parity drive), an in-memory exclusive or > against the new data, and two writes. Reads and writes can be in > parallel. You're right, which means I came to my conclusion wrongly, I guess. :-) Part of this was because I was also thinking about how the array behaves after a failure, as you mention next: > The "work" for parity updates only scales linearly with number of disks > if you use a naive parity algorithm. Or, obviously, if a drive fails. Even using a non-naive :-) algorithm, RAID-5 writes still take somewhat more work than RAID-1 writes do in terms of I/O ops. -- -Chuck
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