iostat busy value calculation

[Slawa Olhovchenkov]

On Wed, Jun 20, 2018 at 07:37:20PM +0200, Miroslav Lachman wrote:

> > %busy comes from the devstat layer. It's defined as the percent of the 
> > time over the polling interval in which at least one transaction was 
> > awaiting completion by the lower layers. It's an imperfect measure of 
> > how busy the drives are (in ye-olden days, before tagged queuing and 
> > NCQ, it was OK because you had THE transaction pending and it was a good 
> > measure of how utilized things were. Now with concurrent I/O in flash 
> > devices, it's only an imperfect approximation).
> 
> Yes, I am aware of this issue. This percentage is just  "is it slightly 
> loaded or heavily loaded" indicator.

for "heavily loaded" use average transaction time and average queue
length