Re: Am I RAIDed or mirrored

[R Bruce Hoffman <bruce.hoffman@xxxxxxxxxxxxxxxxx>]

Right, but if he just adds the fourth, then the parity still remains 
only on the first three... right? So while SAS improves the three disk 
situation, it still doesn't change the parity implementation of raid 
5... right?


On 10/31/2009 01:56 PM, Sue Baker wrote:
> 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.
>
>