Date: Thu, 19 Aug 2004 09:09:50 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Scott Long <scottl@samsco.org> Cc: Wilko Bulte <wb@freebie.xs4all.nl> Subject: Re: RAID-3? Message-ID: <12289.1092899390@critter.freebsd.dk> In-Reply-To: Your message of "Thu, 19 Aug 2004 00:56:23 MDT." <41244F17.9030007@samsco.org>
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In message <41244F17.9030007@samsco.org>, Scott Long writes: >Greg 'groggy' Lehey wrote: >> I can see that as a great advantage, but it's not part of the RAID-3 >> definition, and I can't see why you couldn't expand RAID-5 in a >> similar manner. Am I missing something? >> >> Greg Yes you are missing the complexity of the code to implement it. As far as I know, RaidFrame is the only working implementation of RAID5 with two redundant disks. >Yes, you are! The advantage of RAID-3 is that there are NO >Read-Modify-Write cycles when writing blocks. Period. Zippo. None. >Every write takes exactly the same amount of time. There is no waiting >for data to be read off of any disks. That is why it's nice to >applications that require fixed latency. RAID-3 has no concept of >stripe sizes becuase of this, unlike 4 and 5. > >Scott Well, in RAID3 the stripe size becomes your sectorsize and if you are using a filesystem that demands a particular sectorsize you may be prevented from using RAID3 because of that. UFS/FFS does not have this problem. The other thing is that RAID5 can be made in any configutation from 1+1 to N+1, whereas RAID3 is generally limited to 2^N+X Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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