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Date: Fri, 22 Jun 2018 18:37:50 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Miroslav Lachman <000.fbsd@quip.cz>
Cc: Warner Losh <imp@bsdimp.com>,
 FreeBSD-STABLE Mailing List <freebsd-stable@freebsd.org>
Subject: Re: iostat busy value calculation
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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