System backup
A system backup is the process of backing up the operating system, files and system-specific useful/essential data. [It] primarily ensures that not only the user data in a system is saved, but also the system's state or operational condition. This helps in restoring the system to the last-saved state along with all the selected backup data.
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Using Btrfs snapshots
See Btrfs#Snapshots, #Snapshots and /boot partition, and Snapper.
Using LVM snapshots
See LVM#Snapshots, Create root filesystem snapshots with LVM, and #Snapshots and /boot partition.
Using rsync
See rsync#As a backup utility.
Using tar
See Full system backup with tar.
Using SquashFS
See Full system backup with SquashFS.
Bootable backup
Having a bootable backup can be useful in case the filesystem becomes corrupt or if an update breaks the system. The backup can also be used as a test bed for updates, with the testing repo enabled, etc. If you transferred the system to a different partition or drive and you want to boot it, the process is as simple as updating the backup's /etc/fstab and your boot loader's configuration file.
This section assumes that you backed up the system to another drive or partition, that your current boot loader is working fine, and that you want to boot from the backup as well.
Update the fstab
Without rebooting, edit the backup's fstab by commenting out or removing any existing entries. Add one entry for the partition containing the backup like the example here:
/dev/sdaX / ext4 defaults 0 1
Remember to use the proper device name and filesystem type.
Update the boot loader's configuration file
For Syslinux, all you need to do is duplicate the current entry, except pointing to a different drive or partition.
syslinux.cfg, you can also temporarily edit the menu during boot. When the menu shows up, press the Tab key and change the relevant entries. Partitions are counted from one, drives are counted from zero.For GRUB, it is recommended that you automatically re-generate the main configuration file. If you want to freshly install all GRUB files to somewhere other than /boot, such as /mnt/newroot/boot, use the --boot-directory flag.
Also verify the new menu entry in /boot/grub/grub.cfg. Make sure the UUID is matching the new partition, otherwise it could still boot the old system. Find the UUID of a partition with lsblk:
$ lsblk -no NAME,UUID /dev/sdXY
where /dev/sdXY is the desired partition (e.g. /dev/sdb3). To list the UUIDs of partitions GRUB thinks it can boot, use grep:
# grep UUID= /boot/grub/grub.cfg
First boot
Reboot the computer and select the right entry in the boot loader. This will load the system for the first time. All peripherals should be detected and the empty folders in / will be populated.
Now you can re-edit /etc/fstab to add the previously removed partitions and mount points.
Snapshots and /boot partition
If your file system supports snapshots (e.g., LVM or Btrfs), these will most likely exclude the /boot partition or ESP.
You can copy the boot partition automatically on a kernel update to your root partition with a pacman hook (make sure the hook file is owned by root):
/etc/pacman.d/hooks/95-bootbackup_pre.hook
[Trigger] Operation = Upgrade Operation = Install Operation = Remove Type = Path Target = usr/lib/modules/*/vmlinuz [Action] Depends = rsync Description = Backing up pre /boot... When = PreTransaction Exec = /usr/bin/bash -c 'rsync -a --mkpath --delete /boot/ "/.bootbackup/$(date +%Y_%m_%d_%H.%M.%S)_pre"/'
/etc/pacman.d/hooks/95-bootbackup_post.hook
[Trigger] Operation = Upgrade Operation = Install Operation = Remove Type = Path Target = usr/lib/modules/*/vmlinuz [Action] Depends = rsync Description = Backing up post /boot... When = PostTransaction Exec = /usr/bin/bash -c 'rsync -a --mkpath --delete /boot/ "/.bootbackup/$(date +%Y_%m_%d_%H.%M.%S)_post"/'