Symmetrical vs. asymmetrical disk arrays

There are symmetrical (for example RAID5) and asymmetrical (like RAID4) RAID arrays.

1	2	p
3	p	4
p	5	6

RAID 5

 

1	2	p
3	4	p
5	6	p

RAID 4

As load increases, performance of an asymmetrical array is limited in some particular point. For example, in RAID 4, during a write operation, a disk with parity will be saturated first. In case of a symmetrical RAID 5 array all the member disks are loaded in the same way; therefore there is no specific disk that limits the performance.

From this two consequences follow:

1.   In a symmetrical RAID 5 array write performance can be increased by adding the disks. Write performance of asymmetrical RAID 4 doesn't change as the number of drives increases because parity data is still written to the single disk.

2.   If you add one speedy rotational disk or SSD to a RAID 5 array, you will not get noticeable speed-up. In case of RAID 4, the replacement of the parity disk with an SSD increases performance significantly because "bottleneck" related to parity update is removed.

All this applies in a similar manner to symmetrical RAID 6 and RAID 1E, and asymmetrical RAID3 and RAID-DP.