Building a NAS: part 4 - ZFS mirror

At the end of the previous post I had a ZFS pool backed by a single vdev. This is all well and good until a disk fails. To improve fault tolerance ZFS allows you to add a mirror disk. This obviously isn't a backup; but should make life easier if a disk fails unexpectedly.

Adding a mirror

Assuming you are starting from a pool backed by a single vdev, adding an additional vdev to mirror the content is relatively straightforward.

1. First make sure the pool is online:

   $ zpool status pool: mypool state: ONLINE
     scan: none requested
   config:
   
           NAME        STATE     READ WRITE CKSUM
           mypool      ONLINE       0     0     0
             sdc       ONLINE       0     0     0
   
   errors: No known data errors
   

2. Then identify the device to join to the pool using lsblk:

   $ lsblk
   NAME                MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
   sda                   8:0    0 37.3G  0 disk
   ├─sda1                8:1    0  731M  0 part  /boot
   ├─sda2                8:2    0    1K  0 part
   └─sda5                8:5    0 36.6G  0 part
     └─sdb5_crypt      253:0    0 36.6G  0 crypt
       ├─vghost-root   253:1    0 35.6G  0 lvm   /
       └─vghost-swap_1 253:2    0  976M  0 lvm   [SWAP]
   sdb                   8:16   1  492M  0 disk
   └─sdb1                8:17   1  491M  0 part
   sdc                   8:32   0  3.7T  0 disk
   ├─sdc1                8:33   0  3.7T  0 part
   └─sdc9                8:41   0    8M  0 part
   sdd                   8:48   0  3.7T  0 disk
   ├─sdd1                8:49   0  3.7T  0 part
   └─sdd9                8:57   0    8M  0 part
   

   Note: in this case /dev/sdd is the device we want to use.

3. Clear any existing partitions from the disk (make sure you have the correct disk!):

   $ parted /dev/sdd mklabel gpt
   Warning: The existing disk label on /dev/sdd will be destroyed and all data on this disk will be lost. Do you want to continue?
   Yes/No? yes
   Information: You may need to update /etc/fstab.
   

4. Finally attach the blank disk to the pool, ZFS will automatically start resilvering the new disk to setup a mirror:

   $ zpool attach mypool /dev/sdc /dev/sdd
   
   $ zpool status
     pool: mypool
    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.
     scan: resilver in progress since Mon Apr  6 22:40:24 2020
           38.6G scanned at 963M/s, 432K issued at 10.5K/s, 2.76T total
           0B resilvered, 0.00% done, no estimated completion time
   config:
   
           NAME        STATE     READ WRITE CKSUM
           mypool      ONLINE       0     0     0
             mirror-0  ONLINE       0     0     0
               sdc     ONLINE       0     0     0
               sdd     ONLINE       0     0     0
   
   errors: No known data errors
   

   Eventually ZFS will finish resilvering and the mirror setup will be complete.