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?
  # AUTO_SWITCH=NO
  
  # 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
  # PACKAGE_MANAGER=tdnf
  
  # 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")
  # _ROOT=("PARTUUID A" "PARTUUID B" "/")
  
  # 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.