Introduction: The Evolving Landscape of Android Partitioning

For years, Android modding revolved around familiar partition structures: /boot, /system, /vendor, /data, and /recovery. Flashing a custom ROM or rooting often meant directly interacting with these block devices. However, with the introduction of A/B (Seamless) System Updates and, more significantly, Dynamic Partitions in Android 10+, the underlying architecture has undergone a profound transformation. This shift presents new challenges and necessitates a deeper understanding for anyone looking to modify their Android device, from flashing custom kernels to installing full-blown custom ROMs. This guide will demystify these modern partitioning schemes, detailing their inner workings and providing essential knowledge for modders.

Understanding A/B System-as-Root: Seamless Updates

A/B (also known as Seamless Updates) was introduced to enable software updates to be installed in the background while the device is running, minimizing downtime during reboots. It achieves this by maintaining two identical sets of partitions: Slot A and Slot B.

How A/B Works

• Dual Partition Sets: The device has two complete sets of root partitions (e.g., system_a, vendor_a, boot_a and system_b, vendor_b, boot_b). Only one set is active at any given time.
• Background Updates: When an update is available, the system downloads and applies it to the currently inactive slot. For example, if Slot A is active, the update is installed on Slot B.
• Seamless Transition: After the update is applied, the user is prompted to reboot. During the reboot, the device simply switches to the newly updated slot (Slot B in our example), making the transition almost instantaneous from the user’s perspective. If there’s an issue, the device can revert to the previous working slot.

Implications for Modding with A/B

For modders, A/B means you’re always interacting with an active slot. Tools like fastboot or adb will typically operate on the currently booted slot. When rooting with Magisk, for instance, you’re patching the boot.img of the active slot. If an OTA update comes, it will apply to the inactive slot, and switching to it will mean you lose your root until you re-patch that newly active slot.

# Check current active slot
adb shell bootctl get-current-slot
# Output example: _a

# Set a different slot as active (use with extreme caution!)
# adb shell bootctl set-active-boot-slot _b

The Rise of Dynamic Partitions (Android 10+)

While A/B streamlined updates, Dynamic Partitions, introduced in Android 10, fundamentally altered how storage is managed. Previously, partitions like /system, /vendor, and /product were fixed-size physical partitions on the storage device. Dynamic Partitions consolidate these into a single, large super partition.

What are Dynamic Partitions?

Dynamic Partitions are logical partitions allocated within a larger, static super partition. This allows OEMs to dynamically resize these partitions during OTA updates, offering greater flexibility and efficiency in device storage management. For instance, if a future Android version requires a larger system partition, an OEM can resize it without needing to repartition the entire device.

Why Dynamic Partitions?

• Flexibility: OEMs can adjust partition sizes without changing the partition table, optimizing space usage.
• Project Treble Enhancement: Further decouples the Android framework from vendor implementations, making GSI (Generic System Image) development and flashing more adaptable.
• Reduced Partition Count: Simplifies the overall partitioning scheme.

Challenges for Modders with Dynamic Partitions

The primary challenge is that traditional methods of flashing images to fixed-size partitions (e.g., fastboot flash system system.img) no longer work directly. The system partition isn’t a standalone block device anymore; it’s a logical volume inside super.

# This command will fail on devices with Dynamic Partitions (Android 10+)
# unless fastbootd is running and handling logical partitions.
fastboot flash system system.img

The ‘super’ Partition and Logical Partition Management

At the heart of Dynamic Partitions is the super partition. This is a physical partition that acts as a container for all dynamic, logical partitions (e.g., system, vendor, product, system_ext, odm). Inside the super partition, there’s metadata that describes the layout and sizes of these logical partitions.

Key Concepts:

• Logical Partitions: These are the partitions you interact with (system_a, vendor_a, etc.), but they exist as volumes within super.
• Partition Groups: Dynamic partitions can be grouped (e.g., default or qcom_dynamic). This aids in managing different sets of logical partitions.

To interact with these logical partitions, standard fastboot commands needed an overhaul. This led to the introduction of fastbootd.

Modding in the Dynamic Partitions Era: fastbootd to the Rescue

fastbootd is a new userspace fastboot implementation that runs on the Android device itself, not just in the bootloader. It can parse and understand the structure of the super partition and manage logical partitions. When your device is in fastbootd mode (often referred to as ‘userspace fastboot’), you gain the ability to manipulate logical partitions.

Entering fastbootd

Typically, you enter the standard bootloader/fastboot mode first, then issue a command to transition to fastbootd:

# 1. Reboot to standard bootloader/fastboot mode
adb reboot bootloader

# 2. Once in standard fastboot, reboot into fastbootd
fastboot reboot fastboot

The device may reboot or change screens, often displaying a different fastboot interface or a specific text indicating fastbootd mode.

Manipulating Logical Partitions with fastbootd

With fastbootd, you can now interact with logical partitions. Here are some critical commands:

• List logical partitions:

fastboot getvar all
# Look for entries like: product-l partition-size: 0x...
# or similar related to dynamic partitions

• Delete a logical partition: (e.g., to make space for a larger GSI)

fastboot delete-logical-partition system_a

• Create a logical partition with a specific size (in bytes):

fastboot create-logical-partition system_a 1073741824 # Example: 1GB

• Flash an image to a logical partition: This is the most common use case for GSIs or custom system images. fastboot flash will auto-resize if possible, but fastboot flash:raw is more explicit.

# This command flashes system_a, but fastbootd handles allocating within super
fastboot flash system_a system.img

# Alternatively, for raw flashing (less common for modders)
# fastboot flash:raw system_a system.img

• Wipe a logical partition:

fastboot erase system_a

Rooting with Magisk on Dynamic Partition Devices

Magisk largely abstracts away the complexities of dynamic partitions by focusing on patching the boot.img or init_boot.img (for Android 12+). The process remains similar:

1. Extract the stock boot.img or init_boot.img for your device and current slot.
2. Patch it using the Magisk app.
3. Flash the patched image via fastboot (this still works directly as boot.img is not a dynamic partition):

fastboot flash boot magisk_patched.img
# For Android 12+ on some devices, it might be init_boot
# fastboot flash init_boot magisk_patched.img

After flashing, reboot, and Magisk should be installed. If an OTA update comes, it will update the inactive slot, and you’ll need to repeat the patching process for that slot once it becomes active.

Flashing Custom ROMs and GSIs

Flashing custom ROMs or GSIs on dynamic partition devices often involves using fastbootd. The general steps usually involve:

1. Booting to fastbootd.
2. Wiping existing logical partitions (like system_a, vendor_a) or deleting them to recreate them with appropriate sizes for the GSI.
3. Flashing the new system image to the respective logical partition.
4. Flashing other necessary components like vendor (if a separate vendor image is required).
5. Rebooting the device.

Specific instructions can vary significantly between devices and ROMs, so always consult the device-specific guides. Some advanced ROMs might provide a flashable ZIP that handles these fastbootd operations internally via a custom recovery, but the underlying principle remains the same.

Conclusion: Adapting to the New Modding Paradigm

A/B System-as-Root combined with Dynamic Partitions represents a significant evolution in Android’s architecture. While it makes device updates more robust and efficient, it introduces a steeper learning curve for modders. The key takeaway is understanding that most traditional partitions are now logical volumes within a super partition, and interacting with them requires the use of fastbootd. By mastering these concepts and the associated fastboot commands, modders can continue to explore, customize, and push the boundaries of their Android devices in this new, dynamic era. Always proceed with caution, back up your data, and refer to device-specific guides for the most accurate instructions.