From owner-freebsd-questions@freebsd.org  Wed Aug 30 21:40:56 2017
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Date: Wed, 30 Aug 2017 15:40:54 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: fml <tetrosalame@gmail.com>
cc: freebsd-questions@freebsd.org
Subject: Re: help creating new gmirror > 2TB
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On Wed, 30 Aug 2017, fml wrote:

> On Tue, Aug 29, 2017 at 04:12:12PM -0400, William Dudley wrote:
>> Hi,
>>
>> I want to create a simple mirror > 2TB on a FreeBSD 10.3 system.
>>
>> I have 2 identical 4TB disks.
>
> As you've been told, go with GPT partitioning: disk replacement maybe is
> not as easy as with MBR scheme, so backup your partition table and try
> to rebuild your mirror.
>
> I'm sure you know already: you need enough RAM to fsck such a large
> partition when things go wrong (~700MB per 1TB).

Having done this (and written that article on it), I would say just use 
ZFS.  It is no more fragile than a weird UFS gmirror on GPT.  Some would 
say much less fragile.