Date: Wed, 14 May 2003 11:53:08 +0930 From: Greg 'groggy' Lehey <grog@FreeBSD.org> To: "Marc G. Fournier" <scrappy@hub.org> Cc: freebsd-scsi@freebsd.org Subject: Re: RAID5 capacities / usable drive space ... Message-ID: <20030514022308.GA70087@wantadilla.lemis.com> In-Reply-To: <20030513220947.L3557@hub.org> References: <20030509222154.N728@hub.org> <20030514005737.GA68496@wantadilla.lemis.com> <20030513220947.L3557@hub.org>
next in thread | previous in thread | raw e-mail | index | archive | help
--9jxsPFA5p3P2qPhR Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tuesday, 13 May 2003 at 22:13:45 -0300, Marc G. Fournier wrote: > On Wed, 14 May 2003, Greg 'groggy' Lehey wrote: > >> On Friday, 9 May 2003 at 22:25:51 -0300, The Hermit Hacker wrote: >>> >>> I have someone telling me something that I'd never heard before, and find >>> difficult to believe ... >>> >>> Apparently, he is under the impression that altho a file system shows a >>> capacity of, say, 100G, its usable space is around 50% of that ... >>> anything higher then that, you risk problems ... (significantly reduced >>> MTBF of the drives, degradation in performance, etc) ... >>> >>> His opinion seems to be based on some talks he had with ppl at IBM and >>> Seagate way back in '89, but still seems to feel they are applicable today >>> ... >>> >>> Is there any fact behind his opinion? >> >> It's difficult to say if he hasn't specified reasons. >> >> I can think of a couple of possibilities. One would be, of course, >> that RAID-5 always has overhead for parity, and the other is the fact >> that file system performance deteriorates when the file system fills >> up (thus the 10% left over by UFS). None of these sound like good >> reasons, though. MTBF depends on the activity, not what kind of data >> (allocated/non-allocated) is on the drives. > > 'K ... I'm going to be setting up a server to test my knowledge here, but, > I've had someone tell me: "the fact that you need a minimum of three > drives in Raid 5, so a three drive configuration in Raid5 is not hot > swappable nor will it boot with less than three working drives." > .... Hmm. You know some interesting someones. Yes, it doesn't make sense to have a RAID-5 volume with less than three drives, but in degraded mode it'll run with two. Or it should, bar implementation constraints. And theoretically you could hot swap them. > My understanding was that if I had three drives in a RAID5 > configuration, and one died, the file system would still function > with the 2 drives ... Yes, that's the intention. Greg -- See complete headers for address and phone numbers --9jxsPFA5p3P2qPhR Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.0 (FreeBSD) iD8DBQE+waiMIubykFB6QiMRAr0eAJ9vuhHLWqeRGJ3hg3J2hh0fkc+pWQCdGiD1 RcK4TaU1WtWHVwXXTStr2TM= =KdzE -----END PGP SIGNATURE----- --9jxsPFA5p3P2qPhR--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20030514022308.GA70087>