[OmniOS-discuss] zfs pool 100% busy, disks less than 10%

[Richard Elling]

On Oct 31, 2014, at 7:14 AM, Eric Sproul <eric.sproul at circonus.com> wrote:

> On Fri, Oct 31, 2014 at 2:33 AM, Rune Tipsmark <rt at steait.net> wrote:
> 
>> Why is this pool showing near 100% busy when the underlying disks are doing
>> nothing at all….
> 
> Simply put, it's just how the accounting works in iostat.  It treats
> the pool like any other device, so if there is even one outstanding
> request to the pool, it counts towards the busy%.  Keith W. from
> Joyent explained this recently on the illumos-zfs list:
> http://www.listbox.com/member/archive/182191/2014/10/sort/time_rev/page/3/entry/18:93/20141017161955:F3E11AB2-563A-11E4-8EDC-D0C677981E2F/
> 
> The TL;DR is: if your pool has more than one disk in it, the pool-wide
> busy% is useless.

FWIW, we use %busy as an indicator that we can ignore a device/subsystem when
looking for performance problems. We don't use it as an indicator of problems. In 
other words, if the device isn't > 10% busy, forgetabouddit. If it is more busy, look
in more detail at the meaningful performance indicators.
 -- richard