What is A/B Partitioning?

The basic idea

On a classic (A-only) device, an OTA update stops the phone, writes the new system image, and reboots. If anything fails mid-way, the device is unbootable.

On an A/B device, the running slot (say A) keeps working. The updater writes the new image to the other slot (B) in the background while you use the phone. On next boot, the bootloader switches to slot B. If B fails to boot after a few tries, the bootloader automatically falls back to A — your phone always has a working OS.

Which partitions are duplicated?

Not all partitions are A/B. The duplicated ones are the ones the OTA replaces:

• boot_a / boot_b — kernel + ramdisk
• vendor_boot_a / vendor_boot_b — vendor ramdisk (Android 12+)
• init_boot_a / init_boot_b — init ramdisk (Android 13+)
• dtbo_a / dtbo_b — device tree overlay
• vbmeta_a / vbmeta_b — verified boot metadata
• system_a / system_b — Android system
• vendor_a / vendor_b — OEM vendor files
• product_a / product_b — product-specific apps
• lk_a / lk_b — bootloader (Motorola specifically)

Not duplicated (single copy, shared by both slots): userdata, metadata, persist, modem (sometimes), cache.

How boot picks a slot

The bootloader stores slot metadata in a small region usually called misc or a dedicated BCB (bootloader control block). On each boot:

1. Read the BCB.
2. Pick the slot marked active.
3. Try to load boot_<slot> and chain into Android.
4. If boot fails N times (usually 7), decrement that slot's retry counter.
5. When retry counter hits 0, mark the slot unbootable and switch to the other.

Working with slots in fastboot

Useful fastboot commands:

• fastboot getvar current-slot — show active slot
• fastboot getvar slot-count — 1 (A-only) or 2 (A/B)
• fastboot getvar slot-suffixes — _a,_b
• fastboot set_active a or fastboot --set-active=a — mark slot A active
• fastboot flash boot_a boot.img — flash to specific slot
• fastboot flash --slot=all boot boot.img — flash both slots

Virtual A/B (Android 11+)

Full A/B doubles the flash used by system/vendor/product. On entry-level phones that was unaffordable. Android 11 introduced Virtual A/B: only the small partitions (boot, dtbo, vbmeta) have real A/B copies. System/vendor/product use snapshots via dm-snapshot:

1. OTA writes updates to a copy-on-write area in userdata.
2. Reboot. Snapshot overlay merges COW into system/vendor on first boot.
3. If anything fails, snapshot is discarded. Classic rollback.

From a flashing-tool perspective, Virtual A/B still exposes _a/_b suffixes for the real A/B partitions, but system/vendor/product are no longer per-slot.

FAQ