Date: Thu, 9 Mar 2000 07:24:07 -0800 From: Scott Hess <scott@avantgo.com> To: Greg Lehey <grog@lemis.com> Cc: Scott Hess <scott@AvantGo.com>, james@icorp.net, freebsd-questions@FreeBSD.ORG Subject: Re: RAID/config questions Message-ID: <20000309072406.A10953@river.avantgo.com> In-Reply-To: <20000309141407.L58942@freebie.lemis.com> References: <38C45697.D736070F@icorp.net> <20000306183129.B2525@river.avantgo.com> <20000309141407.L58942@freebie.lemis.com>
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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? 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. 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. The above _did_ happen for the home-grown test suite we were using to replicate our production I/O usage. > >> 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. In general, I agree that you're better off with shared access to a powerful (fast or wide or nearby) resource than dedicated access to a less powerful resource. But unless the controller cannot stream through memory faster than data can be streamed to the disk, it shouldn't make a difference. Personally, I'd never bother going with hardware RAID unless I intended to put a serious amount of disk space on the system in the first place, at which point it makes more sense to buy a decent controller. > > 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. Much of the time you're using RAID because disk I/O is the bottleneck, meaning that the CPU load is besides the point in the first place! At least where I've had to use RAID, I'd have gladly raised the I/O CPU load 10x if that doubled the disk I/Os per second that I could do. Later, scott To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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