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PC-BSD® supports a feature of ZFS known as multiple boot environments (BEs). With multiple boot environments, the process of updating software becomes a low-risk operation as you can backup your current boot environment before upgrading or making software updates to your system. If needed, you also have the option of booting into a backup boot environment. For example:
- if you are making software changes to a boot environment, you can take a snapshot of that environment at any stage during modifications by using the beadm create command. A snapshot is a read-only image of a boot environment at a given point in time. A snapshot is not bootable but you can create a boot environment, based on that snapshot, by using the beadm create -e command followed by the beadm activate command to specify that this boot environment will become the default boot environment on the next reboot.
- you can save multiple boot environments on your system and perform various updates on each of them as needed. For example, you can clone a boot environment by using the beadm create command. A clone is a bootable copy of a boot environment. You can install, test, and update different software packages on the original boot environment and on its clone.
- although only one boot environment can be active at a time, you can mount an inactive boot environment using the beadm mount command. You could then chroot into the mount point in order to update specific packages on the mounted environment.
- you can move a boot environment to another machine, physical or virtual, in order to check hardware support.
for boot environments to work properly, do not change the default ZFS layout during installation. The default ZFS layout ensures that when you create multiple boot environments, the /usr/pbi/, /usr/local/, /usr/home/, /usr/ports/, /usr/src/ and /var/ directories remain untouched. This way, if you rollback to a previous boot environment, you will not lose data in your home directories, any installed applications, or downloaded src or ports.
Managing Boot Environments
Boot environments are managed with the beadm command which must be run as the superuser. The following example creates a BE named beforeupgrade. The new BE is a clone of the current BE, the ZFS environment that you booted into.
To view all BEs, use the list command
BE Active Mountpoint Space Policy Created default NR / 6.05G static 2012-07-09 05:06beforeupgrade - - 1K static 2012-07-10 12:25
- R: active on reboot
- N: active now
- -: inactive
In this example, the current BE is called default, it is active now, and at next reboot; and it is mounted. The newly created beforeupgrade BE exists, but is inactive and unmounted. To activate the new BE:
Activated successfully beadm list BE Active Mountpoint Space Policy Created default N / 64.5K static 2012-07-09 05:06beforeupgrade R - 6.05G static 2012-07-10 12:25
The flags now indicate that the system is currently booted into default, but at next boot the system will boot into beforeupgrade. Only one boot environment can be active at a time.
Figure 8.4a: Managing Boot Environments
Figure 8.4b: Managing GRUB Configuration