Configuring A/B Partition System

You need to create a shadow partition set and configure the A/B partition system to use it for Photon OS updates and modifications.

To use the A/B partition system, ensure the following prerequisites:

  • If you boot with BIOS, only a root filesystem pair is needed. If you boot with UEFI, an EFI partition pair is also needed.

  • In the kickstart configuration file, when you create a partition, set the value of the ab parameter as true to create a shadow partition of the user-defined partition.

    To know more about the kickstart configuration, see the following page: Kickstart Support in Photon OS

    The following example shows how to create a shadow partition mounted at /:

      "partitions": [
                        "disk": "/dev/sda",
                        "mountpoint": "/",
                        "size": 0,
                        "filesystem": "ext4",
                        "ab": true
                        "disk": "/dev/sda",
                        "mountpoint": "/sda",
                        "size": 100,
                        "filesystem": "ext4"
  • Configure the system details of the partitions for A/B update in the following configuration file: /etc/abupdate.conf

    The following template shows how a configuration file looks like:

    # either UEFI, BIOS, or BOTH
    # BOOT_TYPE=<boot type>
    # automatically switch to other partition set after update?
    # automatically finalize the update after a switch?
    # AUTO_FINISH=no
    # can choose to either use tdnf or rpm as a package manager
    # if not specified, tdnf is used
    # Provide information about partition sets
    # PartUUID info can be found with the "blkid" command
    # EFI is needed if booting with UEFI
    # Format: PARTUUID A, PARTUUID B, mount point
    # Example: HOME=("PARTUUID A" "PARTUUID B" "/home")
    # Note that the / partition should be labeled as _ROOT
    # EFI=("PARTUUID A" "PARTUUID B" "/boot/efi")
    # List of all partition sets
    # SETS=( "ROOT" )
    # exclude the following directories/files from being synced
    # note that these directory paths are absolute, not relative to current working directory
    # Format: <set name>_EXCLUDE=( "/dir1/" "/dir2" "/dir3/subdir/file" ... "/dirN/" )
    # Example:
    # HOME_EXCLUDE=( "/mnt" "lost+found" )

You can use the abupdate init command to auto-populate all the fields. However, it is recommended that you manually enter the fields for better accuracy.

Note: Persistent or shared partitions that exist outside the active and inactive partition sets are also supported in the A/B partition system. You need not specify the persistent or shared partitions in the configuration files.

Last modified November 8, 2023: Update (3799256)