Rolling back a bad OS update (boot environments)
Brief introduction to boot environments
FreeBSD has a concept of boot environments.
A ZFS boot environment is a bootable clone of the datasets needed to boot the operating system.
Creating a boot environment before performing an upgrade provides a low-cost safeguard: if there is a problem with the update, the system can be rolled back to the point in time before the upgrade.
The FreeBSD boot loader was rewritten for 12.0 to add BE support. Additionally, the default ZFS layout in the FreeBSD installer understands BEs.
Introduction to beadm
beadm is a small shell utility which you can easily install using the pkg:
beadm helps you manage your boot environments. Let's say you've had a bad system update and one of the OS or Hoster components has stopped working correctly.
That's where beadm comes in.
The FreeBSD is smart enough to automatically create a snapshot of the root filesystem during the freebsd-update fetch install, and to store it as a separate boot environment.
So far - so good, right? Well, not quite. It's a bit tedious to manage the boot environments by hand, so we need beadm to simplify it for us.
Activate an old boot env
Let's activate our old boot environment (because we've realized that something went wrong).
Execute beadm list to show all boot environments that are available to us:
# Example output
[root@hoster-0106 ~]# beadm list
BE Active Mountpoint Space Created
default NR / 5.9G 2023-08-09 19:07
13.2-RELEASE_2023-10-04_092508 - - 1.1G 2023-10-04 09:25
To activate our previous working environment execute the below:
At this point simply reboot, and you'll be using the previous OS version when the system boots up.
More reading
Want to know more? Klara Systems has a really nice and an in-depth article on a similar boot environment management tool bectl: