Earlier this month I wrote about vmstat iowait cpu numbers and some of the comments I got were advertising the use of util% as reported by the iostat tool instead. I find this number even more useless for MySQL performance tuning and capacity planning.

Now let me start by saying this is a really tricky and deceptive number. Many DBAs who report instances of their systems having a very busy IO subsystem said the util% in vmstat was above 99% and therefore they believe this number is a good indicator of an overloaded IO subsystem.

Indeed – when your IO subsystem is busy, up to its full capacity, the utilization should be very close to 100%. However, it is perfectly possible for the IO subsystem and MySQL with it to have plenty more capacity than when utilization is showing 100% – as I will show in an …