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Date: Thu, 18 Jun 2009 23:12:16 -0500
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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Cc: Randy Bush <randy@psg.com>
Subject: Re: adding drive to raidz1
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As a feature suggestion why not reject an "zpool add" of a non-redundant
vdev to a pool of redundant vdev's unless -f is given?  A command of
that sort is almost always a mistake so requiring -f would seem no
hardship for anyone...

Randy Bush wrote:
> 	    NAME        STATE     READ WRITE CKSUM
> 	    tank        ONLINE       0     0     0
> 	      raidz1    ONLINE       0     0     0
> 		ad4s3   ONLINE       0     0     0
> 		ad5s3   ONLINE       0     0     0
> 		ad6s1   ONLINE       0     0     0
> 	      ad7s1     ONLINE       0     0     0
>   

As was said, a vdev (ad7s1) cannot be removed from a pool, and a device
cannot be added to a raidz.

However, I believe it is possible to attach a device to a single-device
vdev such as ad7s1 and turn that vdev into a mirror, regaining
redundancy without recreating the pool, perhaps something like:

# zpool attach tank ad7s1 ad8s1

to get

	    NAME        STATE     READ WRITE CKSUM
	    tank        ONLINE       0     0     0
	      raidz1    ONLINE       0     0     0
		ad4s3   ONLINE       0     0     0
		ad5s3   ONLINE       0     0     0
		ad6s1   ONLINE       0     0     0
              mirror    ONLINE       0     0     0
	        ad7s1   ONLINE       0     0     0
	        ad8s1   ONLINE       0     0     0

(hand edited, not actual zpool output)

Even if the pool the pool is to be rebuilt I suggest converting the naked vdevs to mirrors in the meantime to avoid disaster...

PS. I prefer pools of mirrors over raidz anyway with such a small number of devices since it's easier to protect against many more kinds of system faults (i.e., power supply, cable, device firmware, host controller, driver, etc).