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Date: Wed, 26 Jan 2011 13:58:00 -0600
From: Dan Nelson <dnelson@allantgroup.com>
To: Christer Solskogen <christer.solskogen@gmail.com>
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Cc: "O. Hartmann" <ohartman@mail.zedat.fu-berlin.de>,
	freebsd-questions@freebsd.org
Subject: Re: FreeBSD 9/ZFS: Striped Pool (2 disks) migrating to mirror (onto
 additional disk)
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In the last episode (Jan 26), Christer Solskogen said:
> On Wed, Jan 26, 2011 at 9:42 AM, O. Hartmann wrote:
> > My question is: is it possible to migrate the two-disk pool without data
> > loss into a mirrored pool by adding the one 2TB-disk?
> 
> No, you cant create a two-way mirror of three disks with ZFS. The only way
> of doing what you want by creating a gmirror (or by hardware raid) of the
> two 1TB disks.

You can, if you partition the 2tb disk into two smaller volumes, each the
same size as one of the 1tb disks, then add one of those as a mirror of each
original disk.  You'll end up with two mirrored vdevs in the pool. 
Performance probably won't be as good as a real mirror, though, since zfs
doesn't know that two of its physical disks share a spindle.

Original:

pool1
  da0
  da1

New:

pool1
  mirror
    da0
    da2p1
  mirror
    da1
    da2p2

-- 
	Dan Nelson
	dnelson@allantgroup.com