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Implementing Virtual A/B

To implement virtual A/B on a new device, or to retrofit a launched device, you need to make changes to device-specific code.

Build flags

Devices that use virtual A/B must be configured as an A/B device and must launch with dynamic partitions.

For devices launching with virtual A/B, set them to inherit the virtual A/B device base configuration:

$(call inherit-product, \

Devices launching with virtual A/B need only half as much board size for BOARD_SUPER_PARTITION_SIZE because B slots are no longer in super. That is, BOARD_SUPER_PARTITION_SIZE should be greater than or equal to sum(size of update groups) + overhead, which, in turn, should be greater than or equal to sum(size of partitions) + overhead.

Boot control HAL

The boot control HAL provides an interface for OTA clients to controls boot slots. Virtual A/B requires a minor version upgrade of the boot control HAL because additional APIs are needed to tell bootloader to take care during flashing/factory reset. See IBootControl.hal and types.hal for the latest version of the HAL definition.

// hardware/interfaces/boot/1.1/types.hal
enum MergeStatus : uint8_t {

// hardware/interfaces/boot/1.1/IBootControl.hal
package android.hardware.boot@1.1;
interface IBootControl extends @1.0::IBootControl {
    setSnapshotMergeStatus(MergeStatus status)
        generates (bool success);
        generates (MergeStatus status);
// Recommended implementation

Return<bool> BootControl::setSnapshotMergeStatus(MergeStatus v) {
    // Write value to persistent storage
    // e.g. misc partition (using libbootloader_message)
    // bootloader rejects wipe when status is SNAPSHOTTED
    // or MERGING

Fstab changes

The integrity of the metadata partition is essential to the boot process, especially right after an OTA update is applied. So, the metadata partition should be checked before first_stage_init mounts it. To do so, add the check fs_mgr flag to the entry for /metadata. For example:

/dev/block/by-name/metadata /metadata ext4 noatime,nosuid,nodev,discard,sync wait,formattable,first_stage_mount,check

Kernel requirements

To enable snapshotting, set CONFIG_DM_SNAPSHOT to true.

For devices using F2FS, include the f2fs: export FS_NOCOW_FL flag to user kernel patch to fix file pinning. Additionally, include the f2fs: support aligned pinned file kernel patch as well.

Virtual A/B relies on features added in kernel version 4.3: the overflow status bit in the snapshot and snapshot-merge targets. All devices launching with Android 9 and later should already have kernel version 4.4 or later.

Retrofitting on devices upgrading to Android 11

When upgrading to Android 11, devices that launched with dynamic partitions can optionally retrofit virtual A/B. The update process is mostly the same as for devices launching with virtual A/B, with some minor differences:

  • Location of COW files — For launch devices, the OTA client uses all available empty space in the super partition before using space in /data. For retrofit devices, there's always enough space in the super partition so that the COW file is never created on /data.
  • Build-time feature flags — For devices retrofitting virtual A/B, both PRODUCT_VIRTUAL_AB_OTA and PRODUCT_VIRTUAL_AB_OTA_RETROFIT are set to `true`:
    $(call inherit-product, \
  • Super partition size — Devices launching with virtual A/B can cut BOARD_SUPER_PARTITION_SIZE in half because B slots are no longer in the super partition. Devices retrofitting virtual A/B will keep the old super partition size, so BOARD_SUPER_PARTITION_SIZE is greater than or equal to 2 * sum(size of update groups) + overhead, which in turn is greater than or equal to 2 * sum(size of partitions) + overhead.

Bootloader changes

During the merge step of an update, /data holds the only whole instance of the Android OS. Once the migration starts, the native system, vendor, and product partitions are incomplete until the copy finishes. If the device is factory reset during this process, either by recovery or the Systems settings dialog, then the device would be unbootable.

Before erasing /data, finish the merge in recovery or rollback depending on the device state:

  • If the new build booted successfully before, finish the migration.
  • Otherwise, rollback to the old slot:
    • For dynamic partitions, roll back to the previous state.
    • For static partitions, set the active slot to the old slot.

Both the bootloader and fastbootd can erase the /data partition if the device is unlocked. While fastbootd can force the migration to complete, the bootloader can’t. The bootloader doesn’t know whether or not a merge is in progress, or what blocks in /data constitute the OS partitions. Devices should prevent the user from bricking the device without knowing the risks by doing the following:

  1. Implement the boot control HAL so that the bootloader can read the value set by setSnapshotMergeStatus().
  2. If the merge status is MERGING, or if the merge status is SNAPSHOTTED and the slot has changed to the newly updated slot, then in the bootloader reject requests to wipe userdata, metadata, or the partition storing the merge status.
  3. Implement the fastboot snapshot-update cancel command so that users can signal to the bootloader that they want to bypass this protection mechanism.
  4. Modify custom flashing tools or scripts to issue fastboot snapshot-update cancel when flashing the entire device. This is safe to issue because flashing the entire device removes the OTA. Tooling can detect this command at runtime by implementing fastboot getvar snapshot-update-status. This command helps differentiate between error conditions.


struct VirtualAbState {
    uint8_t StructVersion;
    uint8_t MergeStatus;
    uint8_t SourceSlot;

bool ShouldPreventUserdataWipe() {
    VirtualAbState state;
    if (!ReadVirtualAbState(&state)) ...
    return state.MergeStatus == MergeStatus::MERGING ||
           (state.MergeStatus == MergeStatus::SNAPSHOTTED &&
            state.SourceSlot != CurrentSlot()));

Fastboot tooling changes

Android 11 makes the following changes to the fastboot protocol:

  • getvar snapshot-update-status — Returns the value that the boot control HAL communicated to the bootloader:
    • If the state is MERGING, the bootloader must return merging.
    • If the state is SNAPSHOTTED, the bootloader must return snapshotted.
    • Otherwise, the bootloader must return none.
  • snapshot-update merge — Completes a merge operation, booting to recovery/fastbootd if necessary. This command is valid only if snapshot-update-status is merging, and is only supported in fastbootd.
  • snapshot-update cancel — Sets the boot control HAL's merge status to CANCELLED. This command is invalid when the device is locked.
  • erase or wipe — An erase or wipe of metadata, userdata, or a partition holding the merge status for the boot control HAL should check the snapshot merge status. If the status is MERGING or SNAPSHOTTED, the device should abort the operation.
  • set_active — A set_active command that changes the active slot should check the snapshot merge status. If the status is MERGING, the device should abort the operation. The slot can safely be changed in the SNAPSHOTTED state.

These changes are designed to prevent accidentally making a device unbootable, but they can be disruptive to automated tooling. When the commands are used as a component of flashing all partitions, such as running fastboot flashall, it's recommended to use the following flow:

  1. Query getvar snapshot-update-status.
  2. If merging or snapshotted, issue snapshot-update cancel.
  3. Proceed with flashing steps.

Reducing storage requirements

Devices that don't have full A/B storage allocated in super, and are expecting to use /data as necessary, are strongly recommended to use the block mapping tool. The block mapping tool keeps block allocation consistent between builds, reducing unnecessary writes to the snapshot. This is documented under Reducing OTA Size.

Additional patches

Cherry-pick the following patches to address the following known issues.

Sideloading a full OTA package on a Virtual A/B device that has a super partition with a size smaller than 2 * sum(size of update groups) may fail with the following in recovery log /tmp/recovery.log:

The maximum size of all groups with suffix _b (...) has exceeded half of allocatable space for dynamic partitions ...

Here is an example of the log:

[] Will overwrite existing partitions. Slot Amay be unbootable until update finishes!
[] The maximum size of all groups with suffix _b (2147483648) has exceeded half of allocatable space for dynamic partitions 1073741824.

If you encounter this issue, cherry pick CL 1399393, rebuild and flash the boot partition or recovery partition if the device does not use recovery as boot.