We move on now to RAID levels that provide resiliency balanced with loss of disk space - protection against single disk failures using parity rather than a mirrored copy. RAID 3 and RAID 5 are closely related; both use an approach where 1 disk's worth of data is reserved for parity. RAID 3 stores all parity on one disk; RAID 5 spreads it equally across all disks. Both RAID 3 and RAID 5 require a minimum of 3 disks, and are often most efficient when there are 2n+1 disks in the array (for example 3, 5, 9 or 17).
With a three disk RAID 3 array, ATTO shows us some very promising results:
Indeed, this is pretty much line ball with the 2 disk RAID 0 set (which is what we should expect - the 3 disk RAID 3 set effectively has two data disks in RAID 0 and a third disk for the parity information). Let’s see what HDTach has to say about the 3 disk RAID 0 set:
This is very promising. 160+MBps across the first 70% of the array is plenty of performance, and close enough to the limits we've previously observed that we should be able to hit those limits again. HDTune then, to see the difference between sequential and random access:
We match the sequential performance curve from HDTach, and we average 150MBps across the array. The minimum 106MBps is more than enough to stream HDTV, and RAID 3 is often considered better than RAID 5 for such purposes.
The most we can say is that random IO is no worse than any of the other modes. But it's still no better. At this stage I really have no hope that it would ever improve.
