For RAID4, it is impossible to process two write operations in parallel.
For RAID 5, this probability is not zero unless this is a RAID 5 consisting of three disks. Such a probability in RAID 5 is computed using the formula below:
(N-2)/N * ((N-3)/(N-1))
where N denotes number of disks in RAID 5, including parity.
Let’s see how the probability depends on the number of member disks:
- 30% for RAID 5 of 5 member disks
- 40% for RAID 5 of 6 member disks
- 47% for RAID 5 of 7 member disks
- 53% for RAID 5 of 8 member disks.
It can be seen that the larger number of member disks, the greater probability that two write operations will be proceeded in parallel.
Let’s summarize the above – as for read speed, RAID 4 is very similar to RAID 5, as write speed is concerned – only arrays of three disks are the same while a RAID 5 with more than 3 member disks beats a RAID 4. Actually, RAID4 is a rather unusual array layout, so even those web resources which help configure your array such as www.raid-calculator.com , don’t speak about RAID 4 at all. In my personal opinion it does have some merit.