Every disk in your storage system has a maximum theoretical IOPS value that is based on a formula. Disk performance and IOPS is based on three key factors:

• Rotational speed. Measured in RPM, mostly 7,200, 10,000 or 15,000 RPM. A higher rotational speed is associated with a higher performing disk.
• Average latency. The time it takes for the sector of the disk being accessed to rotate into position under a read/write head.
• Average seek time. The time (in ms) it takes for the hard drive’s read/write head to position itself over the track being read or written.
• Average IOPS: Divide 1 by the sum of the average latency in ms and the average seek time in ms (1 / (average latency in ms + average seek time in ms).

To calculate the IOPS range, use this formula: Average IOPS: Divide 1 by the sum of the average latency in ms and the average seek time in ms (1 / (average latency in ms + average seek time in ms).

Let's calculate the Rotational Delay - RD for a 10K RPM drive:

• Divide 10000 RPM by 60 seconds: 10000/60 = 166 RPS
• Convert 1 of 166 to decimal: 1/166 = 0.006 seconds per Rotation
• Multiply the seconds per rotation by 1000 milliseconds (6 MS per rotation).
• Divide the total in half (RD is considered half a revolution around a disk): 6/2 = 3 MS
• Add an average of 3 MS for seek time: 3 MS + 3 MS = 6 MS
• Add 2 MS for latency (internal transfer): 6 MS + 2 MS = 8 MS
• Divide 1000 MS by 8 MS per I/O: 1000/8 = 125 IOPS

Calculate the rotational delay (RD) for a 5400 RPM drive.