Date: Mon, 29 Oct 2001 10:07:28 +1030 From: Greg Lehey <grog@FreeBSD.org> To: Bruce Evans <bde@zeta.org.au> 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: <20011029100728.D88146@monorchid.lemis.com> In-Reply-To: <20011028224500.A2495-100000@delplex.bde.org>; from bde@zeta.org.au on Sun, Oct 28, 2001 at 10:57:33PM %2B1100 References: <20011027102214.C7091@wantadilla.lemis.com> <20011028224500.A2495-100000@delplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
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. 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). Four such transfers are required for each parity calculation; they should proceed in parallel, which would mean only 27 ms per operation. Assuming your value for memory speed, the 12 kB of memory transfer, by comparison, would take place in 800 µs, nothing by comparison. Greg -- See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe cvs-all" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20011029100728.D88146>