Date: Sun, 6 May 2001 12:52:20 +0930 From: Greg Lehey <grog@lemis.com> To: "Andrew C. Hornback" <hornback@wireco.net> Cc: FreeBSD Questions <questions@FreeBSD.ORG> Subject: Re: Raid Message-ID: <20010506125219.E39554@wantadilla.lemis.com> In-Reply-To: <001f01c0d5d8$3ed3e420$0e00000a@tomcat>; from hornback@wireco.net on Sat, May 05, 2001 at 10:57:11PM -0400 References: <20010506101618.B39554@wantadilla.lemis.com> <001f01c0d5d8$3ed3e420$0e00000a@tomcat>
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On Saturday, 5 May 2001 at 22:57:11 -0400, Andrew C. Hornback wrote: > On Saturday, May 05, 2001 8:46 PM, Greg Lehey wrote: >> On Saturday, May 05, 2001 12:19 PM, Steve Blanzy wrote: >>> On Saturday, 5 May 2001 at 12:34:23 -0400, Andrew C. Hornback wrote: >>>> What RAID if any does free BSD support in a dual SCSI hard >> drive configuration? >>> >> a> (ACK! Please don't send HTML-ized e-mail to the list!) >> >> In fact, his message was multipart-alternative. Your MUA shouldn't >> have any difficulty with that. But please adjust it to make text >> lines of < 80 characters. > > It should have been set to break lines at 76 characters... of > course, it's a Microsoft client... *shakes his head* Guess I should > apologize, eh? Well, not for that. I was talking to Steve. >>>> RAID 1 is pure mirroring, which offers the best redundancy but may >>>> cause a severe penalty on disk subsystem performance. >> >> No, RAID-1 gives you the best performance of any RAID setup. The >> reason why you need at least 3 disks for RAID-5 is because it is >> slower, and though it would theoretically work with only two disks, >> it has no advantages over RAID-1 in this configuration. > > I'm wondering about the explanation here, as I've always been > told (and taught) that RAID 5 offers the best performance of any > RAID level. Don't believe everything you hear. This one is just plain not true. > This may be dependant on a few things (i.e. burst transfer rates of > the drives and the controller), etc. Nope, that has no influence. > RAID 1 causes a performance hit during write, as the data to be > written has to be done twice. Correct, but that can overlap. > RAID 1 is really recommended when you absolutely must have the data > available, because it allows for two copies to be stored. RAID-5 is almost as reliable. > RAID 5, I would imagine, suffers a performance penalty in the > whole comparison with the parity record, but, as I mentioned before, > would perform better if the total burst transfer rate of the drives > was equal to the burst transfer rate of the controller, if the > controller's hardware handled the parity. (My knowledge of exactly > how a RAID controller works is spotty, at best...) No RAID-5 implementation that I know compares parity on reading; read performance is comparable to RAID-1. But on write, you need: - get (read) old contents of block - get (read) old contents of parity block - compute parity - write parity block - write data block This take much longer than any other RAID implementation, with the notable exception of RAID-4, which does the same operations but overloads its parity disk. Take a look at the section "Performance considerations" in vinum(8) for some details. Greg -- When replying to this message, please copy the original recipients. If you don't, I may ignore the reply. For more information, see http://www.lemis.com/questions.html Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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