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
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