Date: Fri, 10 Mar 2000 19:59:58 +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: <20000310195958.E842@freebie.lemis.com> In-Reply-To: <20000309072406.A10953@river.avantgo.com> References: <38C45697.D736070F@icorp.net> <20000306183129.B2525@river.avantgo.com> <20000309141407.L58942@freebie.lemis.com> <20000309072406.A10953@river.avantgo.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thursday, 9 March 2000 at 7:24:07 -0800, Scott Hess wrote: > On Thu, Mar 09, 2000 at 02:14:08PM +1030, Greg Lehey wrote: >> 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. > > Rather than one-line sniping, could you elaborate? I did. See the URL below. > Performance doesn't degrade when rebuilding the hot spare into the > RAID5 array? RAID1+0 isn't comparable in performance to RAID5 for > some tasks? Rebuilding wasn't marginally noticable on the RAID1+0 > array? If you have specific quibbles with my statement, I'd be more > than happy to go go back and do some different tests, if it looks > like we missed something. RAID-5 is very slow on writes. > Note that I didn't say that RAID1+0 was better than RAID5 for all > possible uses, nor that RAID5 became significantly degraded for all > uses, or anything at all about vinum. This has nothing to do with Vinum. It's a basic property of the way RAID-5 works. >>>> 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. > > My assumption is that if you're going to spend the extra money to > purchase a RAID5 controller, you're going to spend enough to > purchase one that isn't doing parity calculations on a 6502. RAID5 > has to touch all the data, but the calculations themselves are not > rocket science - the CPU on the controller only has to stay ahead of > the drives it's writing to. Right, that was my assumption too. It seems that the RAID implementations on hardware controllers are not all as good as you would expect. The relationship between stripe size and performance suggests that hardware RAID controllers rely heavily on cache for their performance. That works fine in the unlikely event that your machine wants to access the disk purely sequentially. In real life, it's much less useful. 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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20000310195958.E842>