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.

#iostat [ -x for extended statistics, -d to display device stastistics only, -m for displaying r/w in MB/s ]
iostat -xdm

#iostat with -p for specific device statistics
iostat -xdm -p sda

• Run iostat -x 1 5.
• Considering the last two outputs provided by the previous command, calculate the efficiency of IOPS for each of them. Does the amount of data written per I/O increase or decrease?

$ df -kh /dev/loop*

We said in the beginning that the disk I/O subsystems are the slowest part of any system. This is why the I/O monitoring is so important, maximizing the performance of the slowest part of a system resulting in an improvement of the performance of the entire system.

Write a script that reads the data into memory and generates a text file 500 times larger, by concatenating the contents of the following novel olivertwist.txt to itself.

Now we want to analyze what is happening with the I/O subsystem during an expensive operation. Monitor the behavior of the system while running your script using vmstat and iostat.