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Date: Fri, 08 Jun 2018 15:16:25 +0100
From: Frank Leonhardt <freebsd-doc@fjl.co.uk>
To: freebsd-questions@freebsd.org
Subject: Re: ZFS mirror keeps going offline/removed
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*** Fixed Top Post - reply at bottom ***

> On Wed, May 30, 2018 at 8:24 AM, Mike Tancsa <mike@sentex.net> wrote:
> 
>> On 5/29/2018 10:29 PM, Aaron wrote:
>> > So recently my ZFS mirror keeps getting both drives marked as removed
>> which
>> >
>> > May 29 19:20:33 darkserver kernel: ada2 at ahcich2 bus 0 scbus2 target 0
>> > lun 0
>> > May 29 19:20:33 darkserver kernel: ada2: <ST4000DM005-2DP166 0001> s/n
>> > ZDH1CTDZ detached
>> 
>> If I am reading this right, the physical drive is disappearing from 
>> the
>> system.
>> Try installing smartmontools
>> and post
>> smartctl -a /dev/ada2
>> 
>> 
>> 
On 2018-05-31 02:14, Aaron wrote:
> Hm. SMART seems fine (see below for ada2 & ada8), but looks fine. I did
> look at camcontrol, don't see anything obvious but strange that ada2 & 
> ada8

I see you're using "Green" drives, and this is most likely your problem. 
Basically, if they go to sleep to conserve power and then don't respond 
quickly enough then ZFS (and gmirror) will decide they're off-line, or 
at least flaky, and detach them.

Regards, Frank.