Messages in this thread | | | Date | Sat, 18 Oct 2003 15:55:21 -0700 | From | jw schultz <> | Subject | Re: Software RAID5 with 2.6.0-test |
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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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