iostat busy value calculation

[Warner Losh]

On Wed, Jun 20, 2018 at 8:55 AM, Miroslav Lachman <000.fbsd at quip.cz> wrote:

> I would like to know how the value of disk "busy" is calculated?
>
> I want to use values from iostat in the monitoring (zabbix).
> iostat shows averages by default and the last column is "busy %", but the
> first output contains averages from the system boot til now.
>

%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).

Warner