Date: Mon, 29 Oct 2001 19:20:38 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Greg Lehey <grog@FreeBSD.org> Cc: Matthew Jacob <mjacob@feral.com>, Doug Rabson <dfr@nlsystems.com>, Luigi Rizzo <rizzo@aciri.org>, John Baldwin <jhb@FreeBSD.org>, Jonathan Lemon <jlemon@FreeBSD.org>, <cvs-all@FreeBSD.org>, <cvs-committers@FreeBSD.org> Subject: Re: RAID-5 parity calculations (was: cvs commit: src/sys/dev/fxp if_fx) Message-ID: <20011029190516.G9442-100000@delplex.bde.org> In-Reply-To: <20011029100728.D88146@monorchid.lemis.com>
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On Mon, 29 Oct 2001, Greg Lehey wrote: > On Sunday, 28 October 2001 at 22:57:33 +1100, Bruce Evans wrote: > > On Sat, 27 Oct 2001, Greg Lehey wrote: > > > >> On Thursday, 25 October 2001 at 15:24:06 -0700, Matt Jacob wrote: > >>> > >>> And the fastest software RAID-V I've known was at NASA/Ames on the > >>> Convex 3280s- they used the otherwise unused vector units for parity > >>> calculations- this gave write performance for a 22 wide stripe on a > >>> terabyte fileystem to be at about 88% of theoretical maximum, which ^^^^^^^^^^^^^^^^^^^^^^^^^^ > >>> sure aint' bad. > >> > >> The parity calculations for RAID-5 are several orders of magnitude > >> faster than the disk accesses. Even on a 486, they took hardly any > >> time. > > > > Actually, a 486 can't possibly have been more than about one order of > > magnitude faster than the disk accesses, since main memory was only > > that much faster (usually less). My 486DX2/66 has 15MB/sec main memory > > and a 2MB/sec disk. It would be possible to upgrade the disk (but not > > the memory). Then the disk would want to transfer at about half an > > order of magnitude faster then the memory. > > My claims are based on measurements, not theory. So are mine. Since we are talking about the theoretical maximum, only certain measurements are relevant. > You're forgetting that most of the transfer time is in positioning. > That's why (in the original message) I mentioned the transfer size. A > 2 MB/s disk is fast for those days; I've seen more like 800 kB/s. Even > accepting your values, the average seek time is 10 ms (check with > rawio if you have a different expectation). Such a disk, doing > transfers of 6 kB, will perform about 75 random transfers per second, > or about 450 kB/s. (By comparison, a disk with 800 kB/s transfer rate > would perform about 57 transfers). I didn't forget this. It's not interesting that the disk can be slowed down by a huge factor by writing tinygrams. I also intentionally didn't mention that main memory speed might not be a factor because the i/o is already pessimized by using PIO. (The extra main memory accesses for parity computations may reduce the main memory accesses for PIO.) Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe cvs-all" in the body of the message
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