From owner-freebsd-questions@freebsd.org  Mon Jun 29 14:19:36 2015
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Date: Mon, 29 Jun 2015 08:19:27 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: Quartz <quartz@sneakertech.com>
cc: Chris Stankevitz <chrisstankevitz@gmail.com>,
 freebsd-questions <freebsd-questions@freebsd.org>
Subject: Re: Corrupt GPT on ZFS full-disks that shouldn't be using GPT
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On Sun, 28 Jun 2015, Quartz wrote:

>> Remember, ZFS
>> leaves space unused at the end of a disk to allow for variations in
>> nominal disk size.
>
> Holy what the heck, no it doesn't! One big issue with zfs is that you CANNOT 
> shrink a pool's size once it's been created, for any reason. You can't remove 
> vdevs, and any replacement disk must bigger or exactly equal in size; even a 
> disk with one less sector and you're SOL. This is my biggest gripe with zfs 
> by far and in fact I just asked freebsd-fs about this less than a week ago 
> wondering if it had been addressed finally (it hasn't).

It's possible I've confused this with something else.  The person who I 
thought told me about this now denies saying anything like that. 
However, there are copies of the ZFS label at the end of the drive that 
might explain the GPT backup not being overwritten.  I have queries in. 
The fact that the backup GPT is still present indicates that ZFS has not 
written to that area, at least so far, and it should be safe to 
overwrite.