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From: Mel Pilgrim <list_freebsd@bluerosetech.com>
Subject: Migrating from raidz2 to mirror
Message-ID: <0b2c709d-3c71-67f9-5af8-5062b0cf857f@bluerosetech.com>
Date: Tue, 7 Nov 2017 15:30:46 -0800
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I currently have this raidz2 configuration:

NAME           STATE     READ WRITE CKSUM
tank           ONLINE       0     0     0
   raidz2-0     ONLINE       0     0     0
     gpt/zfsd0  ONLINE       0     0     0
     gpt/zfsd1  ONLINE       0     0     0
     gpt/zfsd2  ONLINE       0     0     0
     gpt/zfsd3  ONLINE       0     0     0

I want to switch to a mirror configuration on new disks.  My first 
thought was to add the disks to tank as mirrors of the existing raidz2 
plex and use resilvering to transfer everything without downtime.  The 
man page section for the zpool attach command says the existing device 
"cannot be a part of raidz configuration", but I'm not sure what "part 
of" means here.  Does that mean only that I can't attach a mirror to any 
of the individual members of the raidz2 plex (the gpt/zfsd* shown above) 
or does that include the "raidz2-0" vdev?

In other words, is it possible to attach a single disk as a mirror of 
raidz2-0?