Re: Software RAID5 with 2.6.0-test

[jw schultz <jw@pegasys.ws>]

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

		Remember Cernan and Schmitt