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Date:      Wed, 13 May 2015 09:49:43 -0500
From:      Brandon Wandersee <brandon.wandersee@zoho.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: ZFS stripe to raidz1
Message-ID:  <20150513144943.GA3810@WorkBox.Home>
In-Reply-To: <555350E0.5060705@gmail.com>
References:  <8F1341A6-AA91-418D-97EA-FF7F8AC07F6A@jensenwaud.com> <555350E0.5060705@gmail.com>

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On 05/13, Johan Hendriks wrote:
> 
> You can start with two disks and create a stripe accross both disks.
> Keep in mind that if one fails you will loose all your data!
> Later on you can mirror disk one by *attaching* a disk to this vdev. But
> this way you still have no redundancy. If the second disk/vdev fails,
> you still loose all data. You need to *attach* a second disk to the
> second vdev also. That way you can loose a disk per mirrored vdev. So
> you will need 4 disks.
> 
> regards

Just to elaborate a bit on this (for clarity's sake), not only can ZFS not do
what you'd like, but plain-old RAID can't either. In order to create a RAID
array you'd need to mirror one disk to another: the data layout on one disk is
created in a 100% identical manner on the second disk. Because of this, the
storage capacity of the array is limited to that of the smallest of the two
disks. If you tried to create a RAID array between a 4Tb stripe and a 2Tb disk,
you wouldn't get a 4Tb RAID array, but a 2Tb one (leaving one disk unused), and
(at least in the case of traditional RAID) the process of creating it would wipe
data on the 4Tb stripe anyway.

I believe ZFS can create mirrors on live systems without wiping data, but as
John said the  ideal (though more expensive) track is to start with a stripe and
add another *two-disk* stripe later to mirror the first. Or start with RAID1,
add a second two-disk RAID1 array later, back up all data and stripe a new
filesystem across the two RAID arrays.

-- 
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