Booting on a snapshot in read-only mode can be tricky.
An elegant way is to boot this snapshot using overlayfs (included in the kernel ≥ 3.18).
Using overlayfs, the booted snapshot will behave like a live-cd in non-persistent mode.
The snapshot will not be modified, the system will be able to boot correctly, because a writeable folder will be included in the ram.
(no more problems due to /var not open for writing)
Any changes in this system thus started will be lost when the system is rebooted/shutdown.
To do this, it is necessary to modify the initramfs.
This means that any snapshot that does not include this modified initramfs will not be able to benefit from it.
(except for separate boot partitions)
Pacman -S grub-btrfs
Or if you use git
copy the overlay_snap_ro-install file to /etc/initcpio/install/grub-btrfs-overlayfs
copy the overlay_snap_ro-hook file to /etc/initcpio/hooks/grub-btrfs-overlayfs
You must rename the files. (I did it above)
For example :
overlay_snap_ro-install to grub-btrfs-overlayfs
overlay_snap_ro-hook to grub-btrfs-overlayfs
Keep in mind that the files must have exactly the same name to ensure a match.
Edit the file /etc/mkinitcpio.conf
Added hook grub-btrfs-overlayfs at the end of the line HOOKS.
For example :
HOOKS=(base udev autodetect modconf block filesystems keyboard fsck grub-btrfs-overlayfs)
You notice that the name of the hook must match the name of the 2 installed files. (don't forget it)
Re-generate your initramfs
mkinitcpio -P (option -P means, all preset present in /etc/mkinitcpio.d)
Distributions that use Dracut to make their initramfs (many of the Fedora based Distros) simply have to pass either rd.live.overlay.readonly=1 (to boot into the snapshot read only) or rd.live.overlay.overlayfs=1 (to act like a livedisk, that is files can be changed but changes will be lost on the next boot) to their kernel command line in grub.
Grub-btrfs provides the variable GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS to add any command to the kernel command line. Set it to GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS="rd.live.overlay.overlayfs=1" to make snapshots immutable when booted into.
After changing this run sudo /etc/grub.d/41_snapshots-btrfs to generate a new snapshot-submenu with the parameter added.
Refer to your distribution's documentation or contribute to this project to add a paragraph.