Adding Mirrored Disk to Existing ZFS Pool

Great thing about ZFS is that even with a single disk you get some benefits - data integrity being the most important. And all ZFS commands work perfectly well, for example status:

zpool status
   pool: Data.Tertiary``
  state: ONLINE``
 config:``
         NAME                   STATE     READ WRITE CKSUM``
         Data.Tertiary          ONLINE       0     0     0``
           diskid/DISK-XXX.eli  ONLINE       0     0     0``

However, what if one disk is not sufficient any more? It is clear zpool add can be used to create striped pool for higher speeds. And it is clear we can add another device to make a three way mirror. But what if we want to convert solo disk to mirror configuration?

Well, in that case we can get creative with attach command giving it both disks as an argument:

zpool attach Data.Tertiary ^^diskid/DISK-XXX.eli^^ ^^diskid/DISK-YYY.eli^^

After a few seconds, our mirror is created with all our data intact:

zpool status
   pool: Data.Tertiary
  state: ONLINE
 status: One or more devices is currently being resilvered.  The pool will
         continue to function, possibly in a degraded state.
 action: Wait for the resilver to complete.
 config:
         NAME                     STATE     READ WRITE CKSUM
         Data.Tertiary            ONLINE       0     0     0
           mirror-0               ONLINE       0     0     0
             diskid/DISK-XXX.eli  ONLINE       0     0     0
             diskid/DISK-YYY.eli  ONLINE       0     0     0  (resilvering)

PS: Yes, I use encrypted disks from /dev/diskid/ as I used them in previous ZFS examples. If you want plain devices, just use ada0 and companions instead.