Introduction to Android’s Dynamic Partitioning and super_partition

The landscape of Android device storage has undergone significant evolution, moving from rigidly defined, statically allocated partitions to a more flexible, dynamic system. This shift, primarily introduced with Android 10 and further refined, is centered around the concept of the super_partition. Understanding the super_partition is crucial for anyone involved in Android development, custom ROM creation, or deep system customization, as it dictates how system components like system, vendor, and product are allocated and managed.

Before dynamic partitions, each system component resided in its own dedicated, fixed-size partition. This often led to wasted space or insufficient space for larger updates, especially with the growing footprint of Android itself and OEM customizations. The introduction of dynamic partitions, alongside A/B (seamless) updates and the system-as-root scheme, aimed to solve these challenges by allowing more efficient use of physical storage.

Understanding the Super Partition Architecture

The Problem with Fixed Partitions

In older Android versions, the internal storage was carved into numerous partitions like /system, /vendor, /product, /odm, etc., each with a predefined, unchangeable size. If the /system partition needed more space for an update but /vendor had free space, that space couldn’t be reallocated. This inefficiency often forced OEMs to provision generous, often underutilized, partition sizes to future-proof their devices, leading to wasted storage capacity.

The super_partition Solution

The super_partition acts as a single, large physical block device that pools all space intended for dynamic partitions. Instead of having separate physical blocks for system, vendor, etc., these are now ‘logical’ partitions residing within the super_partition. The actual layout and sizes of these logical partitions are defined by metadata stored within the super_partition itself, typically in an lpm.xml file (Logical Partition Manager XML). This allows for flexible resizing, adding, or removing partitions during updates, without needing to reformat the entire device.

System-as-Root and its Implications

The system-as-root change, implemented in Android 9, merged the root filesystem into the system partition. This streamlined the boot process and update mechanisms. When dynamic partitions arrived, they built upon this foundation. With system-as-root and dynamic partitions, the bootloader now reads the super_partition‘s metadata to locate and mount the active system partition (e.g., system_a or system_b in A/B update schemes), which then contains the root filesystem.

Tools of the Trade: Working with super_partition

To effectively interact with super_partition images, a few key command-line tools are essential. These tools are typically found in the Android Open Source Project (AOSP) source tree, specifically within the platform/system/extras/partition_tool directory, or often bundled with firmware extraction utilities:

• lpunpack: Used to extract individual logical partition images (e.g., system.img, vendor.img) from a super.img.
• lpmake: Used to create a super.img from individual logical partition images and an lpm.xml configuration.
• simg2img: Converts sparse Android images (often what lpunpack outputs) into raw, mountable disk images.
• e2fsck & resize2fs: Standard Linux utilities for checking and resizing ext4 filesystems.
• mount: For mounting raw disk images to inspect their contents.

Reading and Extracting Dynamic Partitions

Step 1: Obtain the super.img

The first step is to get the super.img file. There are two primary methods:

1. From Stock Firmware: Many OEMs distribute firmware as a payload.bin file. Tools like payload-dumper-go can extract individual partition images, including super.img, from this payload.
2. From a Running Device: If your device has root access, you can pull the super_partition directly from the device’s block device:

adb shell

        
        
        

            

                
            
            

                
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