Unfortunately it’s still hard to tell who’s wasting most disk IO in too many situations.

Suppose you have two processes – dd and mysqld.

dd is doing massive linear IO and its throughput is 10MB/s. Let’s say dd reads from a slow USB drive and it’s limited to 10MB/s because of the slow reads from the USB.

At the same time MySQL is doing a lot of very small but random IO. A modern SATA 7200 rpm disk drive is only capable of about 90 IO operations per second (IOPS).

So ultimately most of the disk time would be occupied by the mysqld. Still iotop would show dd as the bigger IO user.

– Teodor Milkov