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Date: Tue, 31 Jan 2017 15:01:33 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Sergey Matveychuk <sem33@yandex-team.ru>
cc: freebsd-questions@freebsd.org
Subject: Re: boot from second disk
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On Tue, 31 Jan 2017, Sergey Matveychuk wrote:

> I set up FreeBSD 11.0 on first disk and next I added second disk and I want 
> to do a mirror remotely.
> I followed instructions on 
> https://www.freebsd.org/doc/handbook/geom-mirror.html
> With one difference: I use GPT partitions

gmirror(8) is incompatible with GPT.  Both put their metadata at the end 
of a disk.  There are ugly ways of mirroring partitions, but it is not 
worth it.  Unless the disks are larger than 2TB, create an MBR/disklabel 
layout as shown in the Handbook. If the disks are larger than 2TB, use a 
ZFS mirror.

If the first disk has been set up correctly with bootcode, that will all 
be mirrored to the second disk when the mirror is created. So the system 
will be able to boot from either disk.