Performing an A/B Update

Introduction

Trident can be used to service an operating system, running either on bare metal or virtual machines. To accomplish this, Trident uses an A/B Update strategy.

This document describes how to build an update image, create an update Trident Host Configuration, and execute an update with trident update.

Prerequisites

1. Ensure that oras is installed.
2. Ensure Image Customizer container is accessible.
3. Ensure SSH key pair exists (assumed in this tutorial to be $HOME/.ssh/id_rsa.pub)
4. A bare metal machine (via Hello World) or virtual machine (via Onboard a VM to Trident) has been provisioned.

Instructions

Step 1: Download the Minimal Base Image

Pull minimal-os as a base image from MCR by running:

mkdir -p $HOME/staging
pushd $HOME/staging
oras pull mcr.microsoft.com/azurelinux/3.0/image/minimal-os:latest
popd

Step 2: Build Trident RPMs

Build the Trident RPMs by running:

make bin/trident-rpms.tar.gz

After running this make command, the RPMs will be built and packaged into bin/trident-rpms.tar.gz and unpacked into bin/RPMS/x86_64:

$ ls bin/RPMS/x86_64/
trident-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm
trident-install-service-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm
trident-provisioning-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm
trident-service-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm
trident-static-pcrlock-files-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm
trident-update-poll-0.3.DATESTRING-dev.COMMITHASH.azl3.x86_64.rpm

Copy RPMs to staging folder:

cp -r bin/RPMS $HOME/staging

Step 3: Define Update COSI Configuration

For an update COSI, we need to provide only an esp and the updated partitions. The trident partition does not have an A/B volume pair and does not need to be serviced, so it is not included. The same would go for any data or other non-serviced partition.

Image Customizer reflects the update OS image, which will be laid out onto a single partition at a time: either A or B. So, the A/B volume pairs will not be reflected in the Image Customizer config. This is why root is specified here unlike the root-a or root-b found in the Trident Host Configuration.

  disks:
    - maxSize: 5G
      partitions:
        - id: esp
          size: 8M
          type: esp
        - id: root
          size: 4G

Similarly, filesystems should only contain entries for esp and the updated filesystems:

  filesystems:
    - deviceId: esp
      mountPoint:
        options: umask=0077
        path: /boot/efi
      type: fat32
    - deviceId: root
      mountPoint: /
      type: ext4

The final step for A/B Update is trident commit. This will validate and ensure the machine's boot order is correct after an update. To enable trident commit in the update image, the trident-service package must be installed and the trident service needs to be enabled:

  packages:
    install:
      - trident-service

  services:
    enable:
      - trident

To put all of that together, create the update Image Customization configuration file like this:

cat << EOF > $HOME/staging/ic-config-update.yaml
storage:
  bootType: efi
  disks:
    - maxSize: 5G
      partitionTableType: gpt
      partitions:
        - id: esp
          size: 8M
          type: esp
        - id: root
          size: 4G

  filesystems:
    - deviceId: esp
      mountPoint:
        options: umask=0077
        path: /boot/efi
      type: fat32
    - deviceId: root
      mountPoint: /
      type: ext4

os:
  bootloader:
    resetType: hard-reset
  hostname: updated-testimage

  kernelCommandLine:
    extraCommandLine:
      - rd.info
      - log_buf_len=1M

  packages:
    remove:
      - grub2-efi-binary

    install:
      # replace grub2-efi-binary with grub2-efi-binary-noprefix
      - grub2-efi-binary-noprefix
      - curl
      - dnf
      - efibootmgr
      - iproute
      - iptables
      - jq
      - lsof
      - netplan
      - openssh-server
      - trident-service
      - vim

  services:
    enable:
      - trident
EOF

Step 4: Create an Update COSI

From previous steps, the Trident RPMs, a base image (image.vhdx) and Image Customizer configuration file (ic-config-update.yaml) are all found in $HOME/staging.

Invoke Image Customizer, again paying special attention to specify --output-image-format=cosi:

pushd $HOME/staging
docker run \
    --rm \
    --privileged=true \
    -v /dev:/dev \
    -v ".:/staging:z" \
    mcr.microsoft.com/azurelinux/imagecustomizer:0.18.0 \
        --image-file "/staging/image.vhdx" \
        --config-file "/staging/ic-config-update.yaml" \
        --rpm-source "/staging/RPMS/x86_64" \
        --build-dir "/build" \
        --output-image-format "cosi" \
        --output-image-file "/staging/osimage-update.cosi"
popd

Step 5: Create Trident Host Configuration for Update

To update our existing installation, we need a new Host Configuration file that represents our desired state. In this case, we are only changing the OS based on a new COSI file that was created in step 4. In essence, the Host Configuration file used to deploy the initial operating system can be used as a basis, only changing the COSI file reference:

  image:
      url: /tmp/osimage-update.cosi
      sha384: ignored

Assuming a disk path of /dev/sda and a local COSI file, the Trident update Host Configuration can be created like this:

DISK_DEVICE_PATH="/dev/sda"
cat << EOF > $HOME/staging/host-config-update.yaml
image:
  url: /tmp/osimage-update.cosi
  sha384: ignored
storage:
  disks:
    - id: os
      device: $DISK_DEVICE_PATH
      partitionTableType: gpt
      partitions:
        - id: esp
          type: esp
          size: 1G
        - id: root-a
          type: root
          size: 8G
        - id: root-b
          type: root
          size: 8G
        - id: trident
          type: linux-generic
          size: 1G
  abUpdate:
    volumePairs:
      - id: root
        volumeAId: root-a
        volumeBId: root-b
  filesystems:
    - deviceId: root
      mountPoint: /
    - deviceId: esp
      mountPoint:
        path: /boot/efi
        options: umask=0077
    - deviceId: trident
      source: new
      mountPoint: /var/lib/trident
os:
  selinux:
    mode: enforcing
  netplan:
    version: 2
    ethernets:
      vmeths:
        match:
          name: enp*
        dhcp4: true
  users:
    - name: tutorial-user
      sshPublicKeys:
        - "$(cat $HOME/.ssh/id_rsa.pub)"
      sshMode: key-only
EOF

Step 6: Copy COSI and Host Configuration to the Servicing OS

While Trident can download COSI files from an OCI or http server, in this tutorial, we will just copy the COSI to a known location. This is based on knowing the IP address of the target machine, which can be supplied below as TARGET_MACHINE_IP:

TARGET_MACHINE_IP="<IP ADDRESS>"
# Use SSH Copy to move the update Host Configuration to target machine
scp -i $HOME/.ssh/id_rsa $HOME/staging/host-config-update.yaml tutorial-user@$TARGET_MACHINE_IP:/tmp/host-config-update.yaml
# Use SSH Copy to move the update COSI to target machine
scp -i $HOME/.ssh/id_rsa $HOME/staging/osimage-update.cosi tutorial-user@$TARGET_MACHINE_IP:/tmp/osimage-update.cosi

Step 7: Update OS on the Target Machine to B

To update the target machine, we will invoke trident update.

TARGET_MACHINE_IP="<IP ADDRESS>"
# Use SSH to start an update
ssh -i $HOME/.ssh/id_rsa tutorial-user@$TARGET_MACHINE_IP trident update /tmp/host-config-update.yaml

Conclusion

We have now seen how to build an update image, Trident Host Configuration, and how to invoke update.