Add mirror to single ZFS disk

So you have FreeBSD a single drive ZFS machine and you want to add a second drive to mirror the first because it turns out it’s now important. Yes, it’s possible to do this after installation, even if you’re booting off ZFS.

Let’s assume your first drive is ada0, and it’s had the FreeBSD installer set it up a a “stripe on one drive” using GPT partition. You called the existing zpool “zroot” as you have no imagination whatsoever. In other words everything is the default. The new disk is probably going to be ada1 – plug it in and look on the console or /var/messages to be sure. As long as it’s the same size or larger than the first, you’re good to go. (Use diskinfo -v if you’re not sure).

FreeBSD sets up boot partitions and swap on the existing drive, and you’ll probably want to do this on the new one, if for no other reason than if ada0 fails it can boot off ada1.

gpart destroy -F ada1
gpart backup ada0 | gpart restore ada1
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada1

This gets rid of any old partition table that might be there, copies the existing one from ada0 (which will include the boot and swap partitions as well as the ZFS one).

The third line installs a protective MBR on the disk to avoid non-FreeBSD utilities doing bad things and then adds the ZFS boot code.

If there’s a problem, zero the disk using dd and try again. Make sure you zap the correct drive, of course.

dd if=/dev/zero of=/dev/ada1 bs=32m status=progress

Once you’ve got the partition and boot set up, all you need to do is attach it to the zpool. This is where people get confused as if you do it wrong you may end up with a second vdev rather than a mirror. Note that the ZFS pool is on the third partition on each drive – i.e. adaxp3.

The trick is to specify both the existing and new drives:

zpool attach zroot ada0p3 ada1p3

Run zpool status and you’ll see it (re)silvering the new drive. No interruptions, no reboot.

pool: zroot
  state: ONLINE
   scan: resilvered 677M in 00:00:18 with 0 errors on Sat Apr  5 16:13:16 2025
 config:
     NAME        STATE     READ WRITE CKSUM
     zroot       ONLINE       0     0     0
       mirror-0  ONLINE       0     0     0
         ada0p3  ONLINE       0     0     0
         ada1p3  ONLINE       0     0     0

This only took 18 seconds to resilver as in this case it’s just a system diskm and ZFS doesn’t bother copying unnecessary blocks.

If you want to remove it and go back to a single drive the command is:

zpool detach zroot ada1p3

Add another to create a three-way mirror. Go a little crazy!