RAID 5 is built from four disks, each capable of 200 random input/output operations per second (IOPS). Each small random write requires four I/O operations in total: reading the old data and the old parity, and writing the new data and the new parity; cache effects are ignored. When only writes are performed, which is the number of writes the whole RAID 5 array can process?

Answer: A. 200 per second

The four disks together can process 4 × 200 = 800 I/O operations per second. In RAID 5 each write needs four I/O operations, so the number of writes is 800 ÷ 4 = 200 per second. This increase in the I/O operations needed per write is called the write penalty; for mirrored RAID 10 it is 2, giving 400 per second. For write-heavy uses, the RAID level is chosen considering this performance difference as well as capacity efficiency.

• A. Correct. 800 ÷ 4 = 200 per second.
• B. This is the value for RAID 10, whose write penalty is 2.
• C. This ignores the write penalty and simply adds up the capability of the four disks.