Adding a second disk to the TrueNAS boot pool will increase resilience of the TrueNAS installation in case the original boot device fails - by creating a ZFS mirror (RAID1). This can be easily configured via the web UI or via the CLI. The way of using the command line interface (CLI) is not well documented, so I documented it here.

Note: Make a backup of your TrueNAS configuration before you do any of this - if things go wrong, it will break your TrueNAS installation.

As described in the TrueNAS documentation about mirroring the boot pool, you can configure a boot pool mirror in the web UI by selecting a second disk at System → Boot → Actions → Boot Pool Status → Attach. But I’ve frequently gotten the error message Error: [EFAULT] None when attempting this, and I couldn’t figure out why or how to fix it. It worked however with the CLI.

All the following commands should be executed with root privileges, e.g. via the web UI Shell. First check the current boot pool with the command zpool status boot-pool which should output something like this:

  pool: boot-pool
 state: ONLINE
config:

        NAME        STATE     READ WRITE CKSUM
        boot-pool   ONLINE       0     0     0
          ada0p2    ONLINE       0     0     0

errors: No known data

Adding a mirror disk is a two-step process. Since TrueNAS ZFS mirrors are based on partitions and not entire disks, we first have to create the correct partition table on the new disk. In the following, I want to add the ada1 disk as a mirror to the existing ada0 disk. Clone the partition table from ada0 to ada1 with gpart (effectively backing up the ada0 partition table and restoring it on ada1):

gpart backup ada0 | gpart restore -F ada1

You can then double-check the partition table using gpart show ada1. A TrueNAS 12 system has a bootloader partition (partition 1), a swap partition (partition 3), and the main file system partition (partition 2). Output for a 64 GB boot disk should look like this:

=>       40  125045344  ada0  GPT  (60G)
         40       1024     1  freebsd-boot  (512K)
       1064   33554432     3  freebsd-swap  (16G)
   33555496   91488256     2  freebsd-zfs  (44G)
  125043752       1632        - free -  (816K)

Now we can add the second partition (ada1p2) of the newly formatted disk to the boot pool using zpool attach (the usage is zpool attach pool device new_device). Execute the following command:

zpool attach boot-pool /dev/ada0p2 /dev/ada1p2

Now check the boot pool with zpool status boot-pool. You should see that the pool is currently being resilvered with the status looking like this:

  pool: boot-pool
 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
        boot-pool   ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada0p2  ONLINE       0     0     0
            ada1p2  ONLINE       0     0     0  (resilvering)

errors: No known data errors

Depending on the size of your boot pool, it might take a few minutes until the resilvering is complete. After that, all data will be copied from your original boot disk to the new boot disk. You now have a mirrored boot disk (RAID1)!