CoreOS simple setup for a btrfs time-machine

The aim of this repo is to have a raspberry pi create bi-weekly snapshots of a distant BTRFS subvolume, for a nice off site backup.

Prerequisites

SSH setup

If you want to get the snapshots from a distant sources over SSH some setup is required:

Get the fingerprint of the remote host : ssh-keyscan $hostname >> known_hosts. Note that this could be done in the ignition automatically but blindly accepting the fingerprint is a security risk, do it beforehand and verify it.
Create an ssh key : ssh-keygen -t ecdsa -f id-ecdsa
Upload the public key to the remote host to backup: ssh-copy-id root@$hostname -i id-ecdsa

Build the ignition config

If you have butane installed : butane --pretty --strict --files-dir . config.bu > config.ign

Or with an ephemeral container: podman run --rm -v .:/config/:z quay.io/coreos/butane:release --pretty --strict --files-dir=/config /config/config.bu > transpiled_config.ign

Disk preparation

The ignition config expects a btrfs partition with a label backup. Exemple from a fedora coreOS liveISO :

sudo fdisk /dev/sdX # create the partition
mkfs.btrfs /dev/sdXN

# create a mount point then mount the disk
mkdir /var/backup
mount /dev/sdXN /var/backup
mkfs.btrfs /dev/sdb_vg
# btrbk.conf target subvolume needs to exist
btrfs subvolume create /var/backup/mysubvol

For the raspberry-pi u-boot setup see below, as the disk partitioning setup is different.

Testing

Using cosa: cosa run -i config.ign -D "1G:serial=backup" If so, make sure to add the following butane bit under storage to format the drive:

  filesystems:
    - device: /dev/disk/by-id/virtio-backup
      format: btrfs
      wipe_filesystem: true
      label: backup

Installation

Here we boot the raspberry-pi from a usb disk.
Transpile the config (see above), then install coreOS to the second partition of the disk we just created:

sudo coreos-installer install -s stable -i config.ign backup /dev/sdX

Follow the steps to install u-boot firmware: https://docs.fedoraproject.org/en-US/fedora-coreos/provisioning-raspberry-pi4/#_installing_fcos_and_booting_via_u_boot

For a raspberry-pi 3 some fixing steps are needed : https://discussion.fedoraproject.org/t/fcos-unusually-slow-on-raspberry-pi-3b/40635/16

GDisk will mention some free space, say yes, then create the btrfs partition there with a label backup. I left some space before so the xfs root will be grown to ~20 GB

Be aware that the table partition must be converted to hybrid MBR once again after the first boot. I could do this from the running coreOS system.