initramfs hook for emphemeral root filesystem on btrfs.
/!\ Early stage. Do not use! May lose your data. Seriously, just installing and enabling this system will probably corrupt your Pacman database or get you mismatched kernel and kernel modules versions sooner rather than later. I do use it on a daily basis, but one has to be aware of its warts. Obviously, it comes with no warranty whatoever.
The goal here is to automate as much as possible the use of ephemeral and disposable snapshots as rootfs when running your Linux system on btrfs by restoring the last-committed snapshot of your rootfs the next boot.
# transient info
Rootfs subvolume state: read-only
Last snapshot: 2018-08-21T19:20:57+02:00
Last transient snapshot: 2018-08-21T22:40:21+00:00
Live subvolume state: clean
# touch /etc/foobar
touch: cannot touch '/etc/foobar': Read-only file system
# transient rw
# touch /etc/foobar
# transient info -v
Rootfs subvolume state: writeable
Last snapshot: 2018-08-21T19:20:57+02:00
Last transient snapshot: 2018-08-21T22:40:21+00:00
Live subvolume state: dirty
Created/changed files:
- /etc/foobar
# reboot
-> File will not exist after reboot
# transient rw
# touch /srv/foobar
# transient commit
Create a readonly snapshot of '__live/root' in '__transient/snapshots/2017-04-24T18:16:34+02:00'
Setting rootfs read-only.
# reboot
-> File will be restored from last-commited snapshot
How it works is pretty simple, the live rootfs is deleted (and optionally backed up) on each boot and replaced by the last committed snapshot, effectively erasing all the changes done to the filesystem since the last commit.
A "commit" is simply a btrfs snapshot of your rootfs. At each boot, a backup of the previous rootf can be made to enable recovery (in case you forgot to commit your changes).
The number of backups (done at each boot) and snapshots (done when committing) to be retained can be configured in /etc/btrfs-transient.sh.
I created this package to enable some kind of "transactions" on my Linux systems. Therefore my / is mounted read-only (through btrfs properties), with the start of the transaction being the move to read-write. Data that should not be subject to this transaction process have to get their own subvolume (eg. /home, /srv, maybe /var). One has to be careful with software needing consistent state accross those subvolumes: this software can very easily break that consistency.
$ git clone https://github.com/apognu/btrfs-transient-rootfs.git
$ makepkg
# pacman -U btrfs-transient-0.2-2-any.pkg.tar.xz
Add btrfs-transient to your HOOKS list in /etc/mkinitcpio.conf and rebuild your initramfs:
# vi /etc/mkinitcpio.conf
HOOKS="[...] btrfs btrfs-transient [...]"
# mkinitcpio -p linux
Edit /etc/btrfs-transient.sh and adapt your btrfs device and root subvolume (at least), and reboot.
The first run will:
- create the necessary directories on your btrfs device
- create a first snapshot containing the current state of your root filesystem
- set your root filesystem as read only