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  3. October 2009

Re: Am I RAIDed or mirrored

Lukas Beeler wrote on Sat, 31 Oct 2009 14:14:15 GMT:

That's a non
optimal configuration as three drive RAID5's are notoriously
slow on the IBM i.


I would agree with this statement if the drives were SCSI. But
new SAS technology has changed the rules for parity protection
and RAID5 with 3 drives with SAS rocks!

The biggest difference is how parity is striped across the
drives. In the SCSI days, only 2 of the 3 drives actually had
parity data, so you ended up with what appeared to be 2 drives of
1/2 size and the 3rd drive full size. This could wreak havoc
with performance because the full size drive would get
approximately 2x the disk requests from a system perspective.

With SAS, parity lands on all 3 of the drives in a 3 drive parity
set. So they all are equal in size and from a system perspective
share equally the disk requests.


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