Re: Software RAID5 with 2.6.0-test

jw schultz
2003-10-18T22:55:29Z

On Fri, Oct 17, 2003 at 09:18:24PM +0200, Måns Rullgård wrote:
> Samuel Flory <sflory@rackable.com> writes:
>
> >> What about the RAID controllers in the $400 category? Surely, they
> >> must be doing something better than the $50 fakeraid controllers.
> >>
> >
> > Yes, but follow this logic.
> >
> > 1)You are willing to devote 10% of 2Ghz xeon to software raid.
> > 2)A $500+ controller has a 100Mhz proccessor.
> >
> > Thus just from this you could guess that software raid has x2 as
> > many clock cycles availble to it. It's even worse when you realize
> > the 2Ghz xeon is a better proccessor in many more ways than just
> > clock cycles.
>
> How about this logic:
>
> 1) If the processor on the RAID controller can handle the full
> bandwidth of the disks, it's fast enough.
> 2) If someone else does the 10% work, the CPU can do 10% more work.

And as has been addressed on this list before:

3) If the additional I/O traffic of the RAID can be kept off
of the system busses the overall system throughput goes up.

Once the CPU reaches a certain level of performance it is
the I/O and memory that limit things. Do you really want to
pollute L1 cache with RAID-5? When the $400 RAID server
card can saturate the PCI buss it doesn't matter how much
spare CPU you have, SW RAID will not be able to match the
performance.

--
________________________________________________________________
J.W. Schultz Pegasystems Technologies
email address: jw@pegasys.ws

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