How to add extra LVM volume group disks during the installation

This example is covering the setup of a host with multiple disks and some of them used as part of a LVM setup.

As an example, we have an host with three disks (/dev/sda, /dev/sdb and /dev/sdc).

The first disk is used for a regular Elemental installation and the other remaining two are used as part of a LVM group where arbitrary logical volumes are created, formatted and mounted at boot time via an extended fstab file.

For this example cloud-config steps are required in two different stages. First, some installation hooks are needed to prepare and handle LVM volumes during the installation. Second, a cloud-config is required at boot time to ensure the created LVM volumes are included in /etc/fstab and consequently mounted.

Installation hooks can be included to the SeedImage.spec.cloud-config section with something like:

apiVersion: elemental.cattle.io/v1beta1
kind: SeedImage
metadata:
  name: custom-seed
  namespace: fleet-default
spec:
  ...
  cloud-config:
    name: "Create LVM logic volumes over some physical disks"
    stages:
      post-install:
      - name: "Create physical volume, volume group and logical volumes"
        if: '[ -e "/dev/sdb" ] && [ -e "/dev/sdc" ]'
        commands:
        - | 
          # Create the physical volume, volume group and logical volumes
          pvcreate /dev/sdb /dev/sdc
          vgcreate elementalLVM /dev/sdb /dev/sdc
          lvcreate -L 8G -n elementalVol1 elementalLVM
          lvcreate -l 100%FREE -n elementalVol2 elementalLVM
          # Trigger udev detection
          if [ ! -e "/dev/elementalLVM/elementalVol1" ] || [ ! -e "/dev/elementalLVM/elementalVol2" ]; then
            sleep 10
            udevadm settle
          fi
          # Ensure devices are already available
          [ -e "/dev/elementalLVM/elementalVol1" ] || exit 1
          [ -e "/dev/elementalLVM/elementalVol2" ] || exit 1
          # Format logical volumes with a known label for later use in fstab
          mkfs.xfs -L eVol1 /dev/elementalLVM/elementalVol1
          mkfs.xfs -L eVol2 /dev/elementalLVM/elementalVol2

The LVM devices are created and formatted as desired. This is a good example of an installation hook, as this setup is only needed once, at installation time. As an alternative, the same action could be done on first boot, however it would require a more sophisticated logic to ensure it's only applied once at first boot.

Finally, the boot time cloud-config data contains the mount point settings to trigger the mounts. The Elemental OS fstab file is ephemeral and it's dynamically created at boot time. That's why it doesn't exist during the installation and can't be used in an installation hook.

Consider the following example to customize the /etc/fstab file using the cloud-config section of a MachineRegistration resource:

apiVersion: elemental.cattle.io/v1beta1
kind: MachineRegistration
metadata:
  name: my-nodes
  namespace: fleet-default
spec:
  ...
  config:
    ...
    cloud-config:
      stages:
        initramfs:
        - name: "Extend fstab to mount LVM logical volumes at boot"
          commands:
          - |
            echo "LABEL=eVol1 /run/elemental/eVol1  xfs defaults  0 0" >> /etc/fstab
            echo "LABEL=eVol2 /run/elemental/eVol2  xfs defaults  0 0" >> /etc/fstab

note

The initramfs stage is the last stage before switching to the actual root tree. At this stage, the /etc/fstab file already exists and can be adapted before switching root. Once running in the final root tree, SystemD will handle the rest of the initialization and apply it.