RAID 5 15 disks

With 4+1 the Clariion can perform full stripe writes, while with 14+1 this is almost impossible. A 4+1 Raid Group has the capacity of 4 data disks, but  performs like 5, while a 14+1 has 14 data disks and performs like 15. Needlessly to say that the 4+1 is relatively better.

Furthermore a rebuild to a hot spare in a 4+1 only needs to read data from 4 disks to be able to rebuild the data where a rebuild on the 14+1 needs 14 drives to rebuild number 15. So here the 4+1 wins again.

Now you might say: 15 disks outperform a 4+1. This is true, but I would compare a 14+1 to three 4+1 RGs. If you then create LUNs on the (4+1)s, use METALUNs with 3 components, so you'll have the performance of 15 drives on the LUN.

The only reason why one would use the 14+1 is perhaps cost. You only have 1 drive for parity, which is cheaper than 3 if you decided to create 3 RGs of (4+1) out of the 15 drives. Plus 14+1 gives you the capacity of 14 drives where three times a (4+1) only gives you 3x4=12 drives of capacity.

EMC calls it (4+1) RG the performance RG and a (8+1) the poor mans RG. (14+1) is even worse....