Date: Thu, 9 Mar 2000 14:14:08 +1030 From: Greg Lehey <grog@lemis.com> To: Scott Hess <scott@avantgo.com> Cc: james@icorp.net, freebsd-questions@FreeBSD.ORG Subject: Re: RAID/config questions Message-ID: <20000309141407.L58942@freebie.lemis.com> In-Reply-To: <20000306183129.B2525@river.avantgo.com> References: <38C45697.D736070F@icorp.net> <20000306183129.B2525@river.avantgo.com>
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On Monday, 6 March 2000 at 18:31:29 -0800, Scott Hess wrote: > On Mon, Mar 06, 2000 at 07:08:39PM -0600, James wrote: >> 2. Which will give me better performance? RAID or RAID5? I >> know RAID5 will give me more disk space, but is there any significant >> i/o performance cost? > > My most recent experience was with an external SCSI-SCSI RAID controller > attached to an Adaptec differential host adapter on FreeBSD3.3. With > six drives plus a hot-spare, I found that RAID5 (5 drives plus parity) > and RAID1+0 (3 drives worth of mirrorred and striped data), the performance > was pretty similar. The deciding factor was that when we pulled a drive, > rebuilding under RAID5 really degraded performance (it could carry about > 1/3 the tps relative to when it wasn't degraded), while rebuilding under > RAID1+0 was only marginally noticable. I'd guess you didn't do any particularly rigorous performance testing. >> And generally speaking, how much of a performance degredation may I >> see (if any) in going from a non-raid SCSI to a RAID or RAID 5 setup? > > Hardware RAID shouldn't degrade performance at all. I suppose that depends on what you mean by "hardware RAID". With conventional RAID controllers, the difference is really where the software runs. In "software RAID" it runs in the main CPU, usually quite a powerful processor. On RAID board the processor is usually much slower. On top of that, RAID-5 requires many more I/O accesses for a write than RAID-1 does. This slows down writes, and there's not much you can do about it. > Software RAID0 or RAID1 shouldn't have much impact, RAID5 can have > more because it has to actually muck with the data. Software RAID doesn't pose much of a load on normal modern CPUs. Greg -- When replying to this message, please copy the original recipients. 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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