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Date: Tue, 13 Nov 2012 16:56:54 +0100
From: Miroslav Lachman <000.fbsd@quip.cz>
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To: Jason Keltz <jas@cse.yorku.ca>
Subject: Re: RHEL to FreeBSD file server
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Jason Keltz wrote:
[...]

> Presently, with the mirrors, I'm using 22 disks with leaving 2 hot spares.
> If I used 6 x 4 disk vdevs, I use all the disks and don't have hot
> spares (though I have better redundancy) -- I could put spares into the
> R720 head which actually has 14 disk slots empty, but I'm hesitant to
> put a spare for the md1220 using a different driver in the head of the
> R720...

Beware of hot spares - they are not "hot".

kern/134491: [zfs] Hot spares are rather cold...
http://www.freebsd.org/cgi/query-pr.cgi?pr=134491

This is long standing unsolved issue. You can add drives as "hot spares" 
to the zpool, but FreeBSD lacks daemon to recieve notifications about 
disk failure and failed drive will not be replaced by spare.

Miroslav Lachman