Understanding Partitions in AOSP

What is a Partition?

A partition is a logical division of physical storage inside a device. Instead of treating the entire storage as one block, Android divides it into multiple independent sections. Each partition has a specific purpose, stores specific types of data, and has its own size, filesystem, and mount point.

• ▸/system - Android framework and core OS files
• ▸/vendor - Hardware-specific HAL implementations and binaries
• ▸/data - User apps, settings, and files
• ▸One physical storage chip is divided into multiple independent sections, each serving a distinct role

Storage Technologies: eMMC vs UFS

eMMC (embedded MultiMediaCard)

• ▸Older storage technology with slower performance
• ▸Handles one operation at a time (sequential access)
• ▸Commonly used in budget and older devices
• ▸Reliable, but limited throughput

UFS (Universal Flash Storage)

• ▸Modern storage technology with SSD-like performance
• ▸Supports parallel read and write operations simultaneously
• ▸Used in modern smartphones and most automotive-grade Android systems
• ▸Significantly faster than eMMC for both sequential and random access

Types of Partitions in AOSP

Core System Partitions

• ▸boot - Kernel and ramdisk; used for the boot process
• ▸system - Android framework, core services, and platform apps
• ▸vendor - HALs and vendor-specific binaries for hardware abstraction
• ▸product - OEM-specific customizations and preloaded apps
• ▸odm - Device manufacturer specific components, separate from vendor

Data and Runtime Partitions

• ▸userdata (/data) - User-installed apps, accounts, settings, and personal files
• ▸cache - Temporary data storage; largely deprecated in modern Android
• ▸metadata - Encryption metadata and filesystem state

Special Partitions

• ▸recovery - Recovery mode image for OTA and factory reset operations
• ▸persist - Critical calibration data that survives factory reset
• ▸misc - Boot control flags and miscellaneous system state

Dynamic Partitions (Modern AOSP)

• ▸Grouped inside a single 'super' partition on the physical storage
• ▸Includes system, vendor, product, and odm by default
• ▸Allows flexible resizing of individual partitions without changing the physical layout
• ▸Introduced in Android 10 as part of Project Treble improvements

The Persist Partition

The persist partition stores critical hardware calibration data that must survive a factory reset. While /data is wiped on reset, /persist is preserved. This ensures hardware continues functioning correctly after the device is reset to factory state.

• ▸Camera calibration data specific to the hardware module
• ▸Audio DSP tuning and EQ profiles
• ▸Sensor calibration offsets (accelerometer, gyroscope, etc.)
• ▸Wi-Fi and Bluetooth hardware configuration
• ▸Automotive-specific configs such as VHAL property defaults and ECU pairing data

Key AOSP Files for Partition Management

1. BoardConfig.mk

Defines the partition size and filesystem type. This is where physical dimensions are declared.

  BOARD_MY_CUSTOMIMAGE_PARTITION_SIZE := 104857600
  BOARD_MY_CUSTOMIMAGE_FILE_SYSTEM_TYPE := ext4

2. Android.bp

Defines how the partition image is built during the Android build process.

  android_filesystem {
      name: "my_customimage",
      partition_name: "my_custom",
      mount_point: "/my_custom",
  }

3. fstab (File System Table)

Tells Android where and how to mount each partition at boot time.

  /dev/block/by-name/my_custom  /my_custom  ext4  defaults  wait

4. sepolicy (SELinux)

Defines SELinux security labels and access rules for the partition contents.

  /my_custom(/.*)?  u:object_r:my_custom_file:s0

5. super_partition.xml (Dynamic Partitions)

Defines whether a partition exists inside the super partition and its allocated size.

  <partition name="my_custom" size="104857600"/>

How to Create a Custom Partition in AOSP

• ▸Step 1: Define partition size in BoardConfig.mk - BOARD_MY_CUSTOMIMAGE_PARTITION_SIZE and FILE_SYSTEM_TYPE
• ▸Step 2: Create the partition image definition in Android.bp using android_filesystem{}
• ▸Step 3: Add the mount point entry in the device fstab file
• ▸Step 4: Add SELinux labels and access rules in sepolicy
• ▸Step 5: Register the partition in super_partition.xml if using dynamic partitions

Step 6: Build and Flash

  m my_customimage
  fastboot flash my_custom my_custom.img

How to Add Content to a Custom Partition

During Build (via PRODUCT_COPY_FILES)

  PRODUCT_COPY_FILES += \
    device/<vendor>/<device>/file.txt:my_custom/file.txt

At Runtime (via C/C++ code)

  FILE *fp = fopen("/my_custom/data.txt", "w");
  fprintf(fp, "Hello");
  fclose(fp);