Implementing Dynamic Partitions

Dynamic partitioning is implemented using the dm-linear device-mapper module in the Linux kernel. The super partition contains metadata listing the names and block ranges of each dynamic partition within super. During first-stage init, this metadata is parsed and validated, and virtual block devices are created to represent each dynamic partition.

When applying an OTA, dynamic partitions are automatically created, resized, or deleted as needed. For A/B devices, there are two copies of the metadata, and changes are applied only to the copy representing the target slot.

Because dynamic partitions are implemented in userspace, partitions needed by the bootloader can't be made dynamic. For example, boot, dtbo, and vbmeta are read by the bootloader, and so must remain as physical partitions.

Each dynamic partition can belong to an update group. These groups limit the maximum space that partitions in that group can consume. For example, system and vendor can belong to a group that restricts the total size of system and vendor.

Implementing dynamic partitions on new devices

This section details how to implement dynamic partitions on new devices launching with Android 10 and higher. To update existing devices, see Upgrading Android devices.

Partitioning changes

For devices launching with Android 10, create a partition called super. The super partition handles A/B slots internally, so A/B devices don't need separate super_a and super_b partitions. All read-only AOSP partitions that aren't used by the bootloader must be dynamic and must be removed from the GUID Partition Table (GPT). Vendor-specific partitions don't have to be dynamic and may be placed in the GPT.

To estimate the size of super, add the sizes of the partitions being deleted from the GPT. For A/B devices, this should include the size of both slots. Figure 1 shows an example partition table before and after converting to dynamic partitions.

Partition table layout
Figure 1. New physical partition table layout when converting to dynamic partitions

The supported dynamic partitions are:

  • System
  • Vendor
  • Product
  • System Ext
  • ODM

For devices launching with Android 10, the kernel command line option androidboot.super_partition must be empty so that the command sysprop ro.boot.super_partition is empty.

Partition alignment

The device-mapper module may operate less efficiently if the super partition is not properly aligned. The super partition MUST be aligned to the minimum I/O request size as determined by the block layer. By default, the build system (via lpmake, which generates the super partition image), assumes that a 1 MiB alignment is sufficient for every dynamic partition. However, vendors should ensure that the super partition is properly aligned.

You can determine the minimum request size of a block device by inspecting sysfs. For example:

# ls -l /dev/block/by-name/super
lrwxrwxrwx 1 root root 16 1970-04-05 01:41 /dev/block/by-name/super -> /dev/block/sda17
# cat /sys/block/sda/queue/minimum_io_size
786432

You can verify the super partition's alignment in a similar manner:

# cat /sys/block/sda/sda17/alignment_offset

The alignment offset MUST be 0.

Device configuration changes

To enable dynamic partitioning, add the following flag in device.mk:

PRODUCT_USE_DYNAMIC_PARTITIONS := true

Board configuration changes

You're required to set the size of the super partition:

BOARD_SUPER_PARTITION_SIZE := <size-in-bytes>

On A/B devices, the build system throws an error if the total size of dynamic partition images is more than half of the super partition size.

You can configure the list of dynamic partitions as follows. For devices using update groups, list the groups in the BOARD_SUPER_PARTITION_GROUPS variable. Each group name then has a BOARD_group_SIZE and BOARD_group_PARTITION_LIST variable. For A/B devices, the maximum size of a group should cover only one slot, as the group names are slot-suffixed internally.

Here's an example device that places all partitions into a group called example_dynamic_partitions:

BOARD_SUPER_PARTITION_GROUPS := example_dynamic_partitions
BOARD_EXAMPLE_DYNAMIC_PARTITIONS_SIZE := 6442450944
BOARD_EXAMPLE_DYNAMIC_PARTITIONS_PARTITION_LIST := system vendor product

Here's an example device that places system and product services into group_foo, and vendor, product, and odm into group_bar:

BOARD_SUPER_PARTITION_GROUPS := group_foo group_bar
BOARD_GROUP_FOO_SIZE := 4831838208
BOARD_GROUP_FOO_PARTITION_LIST := system product_services
BOARD_GROUP_BAR_SIZE := 1610612736
BOARD_GROUP_BAR_PARTITION_LIST := vendor product odm
  • For A/B launch devices, the sum of maximum sizes of all groups must be:
    BOARD_SUPER_PARTITION_SIZE / 2 - overhead
  • For non-A/B devices and retrofit A/B devices, the sum of maximum sizes of all groups must be:
    BOARD_SUPER_PARTITION_SIZE - overhead
  • At build time, the sum of the sizes of the images of each partition in an update group must not exceed the maximum size of the group.
  • Overhead is required in the computation to account for metadata, alignments, and so on. A reasonable overhead is 4 MiB, but you can pick a larger overhead as needed by the device.

Sizing dynamic partitions

Before dynamic partitions, partition sizes were over-allocated to ensure that they had enough room for future updates. The actual size was taken as is and most read-only partitions had some amount of free space in their file system. In dynamic partitions, that free space is unusable and could be used to grow partitions during an OTA. It's critical to ensure that partitions aren't wasting space and are allocated to a minimum possible size.

For read-only ext4 images, the build system automatically allocates the minimum size if no hardcoded partition size is specified. The build system fits the image so that the file system has as little unused space as possible. This ensures that the device doesn't waste space that can be used for OTAs.

Additionally, ext4 images can be further compressed by enabling block- level deduplication. To enable this, use the following configuration:

BOARD_EXT4_SHARE_DUP_BLOCKS := true

If automatic allocation of a partition minimum size is undesirable, there are two ways to control the partition size. You can specify a minimum amount of free space with BOARD_partitionIMAGE_PARTITION_RESERVED_SIZE, or you can specify BOARD_partitionIMAGE_PARTITION_SIZE to force dynamic partitions to a specific size. Neither of these is recommended unless necessary.

For example:

BOARD_PRODUCTIMAGE_PARTITION_RESERVED_SIZE := 52428800

This forces the file system in product.img to have 50 MiB of unused space.

System-as-root changes

Devices launching with Android 10 must not use system-as-root.

Devices with dynamic partitions (whether it launches with or retrofits dynamic partitions) must not use system-as-root. The Linux kernel can't interpret the super partition and so can't mount system itself. system is now mounted by first-stage init, which resides in the ramdisk.

Don't set BOARD_BUILD_SYSTEM_ROOT_IMAGE. In Android 10, the BOARD_BUILD_SYSTEM_ROOT_IMAGE flag is only used to differentiate whether the system is mounted by the kernel or by the first-stage init in ramdisk.

Setting BOARD_BUILD_SYSTEM_ROOT_IMAGE to true causes a build error when PRODUCT_USES_DYNAMIC_PARTITIONS is also true.

When BOARD_USES_RECOVERY_AS_BOOT is set to true, the recovery image is built as boot.img, containing the recovery's ramdisk. Previously, bootloader used the skip_initramfs kernel command line parameter to decide which mode to boot into. For Android 10 devices, the bootloader MUST NOT pass skip_initramfs to the kernel command-line. Instead, bootloader should pass androidboot.force_normal_boot=1 to skip recovery and boot normal Android.

AVB configuration changes

When using Android Verified Boot 2.0, if the device isn't using chained partition descriptors, then no change is necessary. If using chained partitions, however, and one of the verified partitions is dynamic, then changes are necessary.

Here's an example configuration for a device that chains vbmeta for the system and vendor partitions.

BOARD_AVB_SYSTEM_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
BOARD_AVB_SYSTEM_ALGORITHM := SHA256_RSA2048
BOARD_AVB_SYSTEM_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
BOARD_AVB_SYSTEM_ROLLBACK_INDEX_LOCATION := 1

BOARD_AVB_VENDOR_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
BOARD_AVB_VENDOR_ALGORITHM := SHA256_RSA2048
BOARD_AVB_VENDOR_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
BOARD_AVB_VENDOR_ROLLBACK_INDEX_LOCATION := 1

With this configuration, the bootloader expects to find a vbmeta footer at the end of the system and vendor partitions. Because these partitions are no longer visible to the bootloader (they reside in super), two changes are needed.

  • Add vbmeta_system and vbmeta_vendor partitions to the device's partition table. For A/B devices, add vbmeta_system_a, vbmeta_system_b, vbmeta_vendor_a, and vbmeta_vendor_b. If adding one or more of these partitions, they should be the same size as the vbmeta partition.
  • Rename the configuration flags by adding VBMETA_ and specify which partitions the chaining extends to:
    BOARD_AVB_VBMETA_SYSTEM := system
    BOARD_AVB_VBMETA_SYSTEM_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
    BOARD_AVB_VBMETA_SYSTEM_ALGORITHM := SHA256_RSA2048
    BOARD_AVB_VBMETA_SYSTEM_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
    BOARD_AVB_VBMETA_SYSTEM_ROLLBACK_INDEX_LOCATION := 1
    
    BOARD_AVB_VBMETA_VENDOR := vendor
    BOARD_AVB_VBMETA_VENDOR_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
    BOARD_AVB_VBMETA_VENDOR_ALGORITHM := SHA256_RSA2048
    BOARD_AVB_VBMETA_VENDOR_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
    BOARD_AVB_VBMETA_VENDOR_ROLLBACK_INDEX_LOCATION := 1
    

A device may be using one, both, or none of these partitions. Changes are need only when chaining to a logical partition.

AVB bootloader changes

If the bootloader has embedded libavb, include the following patches:

If using chained partitions, include an additional patch:

  • 49936b4c0109411fdd38bd4ba3a32a01c40439a9 — "libavb: Support vbmeta blobs in beginning of partition."

Kernel command line changes

A new parameter, androidboot.boot_devices, must be added to the kernel command line. This is used by init to enable /dev/block/by-name symlinks. It should be the device path component to the underlying by-name symlink created by ueventd, that is, /dev/block/platform/device-path/by-name/partition-name.

For example, if the super partition by-name symlink is /dev/block/platform/soc/100000.ufshc/by-name/super, you can add the command line parameter in the BoardConfig.mk file as follows:

BOARD_KERNEL_CMDLINE += androidboot.boot_devices=soc/100000.ufshc

fstab changes

The device tree and device tree overlays must not contain fstab entries. Use an fstab file that will be part of the ramdisk.

Changes must be made to the fstab file for logical partitions:

  • The fs_mgr flags field must include the logical flag and the first_stage_mount flag, introduced in Android 10, which indicates that a partition is to be mounted in the first stage.
  • A partition may specify avb=vbmeta partition name as an fs_mgr flag and then the specified vbmeta partition is initialized by first stage init before attempting to mount any devices.
  • The dev field must be the partition name.

The following fstab entries set system, vendor, and product as logical partitions following the above rules.

#<dev>  <mnt_point> <type>  <mnt_flags options> <fs_mgr_flags>
system   /system     ext4    ro,barrier=1        wait,slotselect,avb=vbmeta,logical,first_stage_mount
vendor   /vendor     ext4    ro,barrier=1        wait,slotselect,avb,logical,first_stage_mount
product  /product    ext4    ro,barrier=1        wait,slotselect,avb,logical,first_stage_mount

Copy the fstab file into the first stage ramdisk.

SELinux changes

The super partition block device must be marked with the label super_block_device. For example, if the super partition by-name symlink is /dev/block/platform/soc/100000.ufshc/by-name/super, add the following line to file_contexts:

/dev/block/platform/soc/10000\.ufshc/by-name/super   u:object_r:super_block_device:s0

fastbootd

The bootloader (or any non-userspace flashing tool) doesn't understand dynamic partitions, so it can't flash them. To address this, devices must use a user-space implementation of the fastboot protocol, called fastbootd.

For more information on how to implement fastbootd, see Moving Fastboot to User Space.

adb remount

For developers using eng or userdebug builds, adb remount is extremely useful for fast iteration. Dynamic partitions pose a problem for adb remount because there is no longer free space within each file system. To address this, devices can enable overlayfs. As long as there is free space within the super partition, adb remount automatically creates a temporary dynamic partition and uses overlayfs for writes. The temporary partition is named scratch, so don't use this name for other partitions.

For more information on how to enable overlayfs, see the overlayfs README in AOSP.

Upgrading Android devices

If you upgrade a device to Android 10, and want to include dynamic partitions support in the OTA, you don't need to change the built-in partition table. Some extra configuration is required.

Device configuration changes

To retrofit dynamic partitioning, add the following flags in device.mk:

PRODUCT_USE_DYNAMIC_PARTITIONS := true
PRODUCT_RETROFIT_DYNAMIC_PARTITIONS := true

Board configuration changes

You're required to set the following board variables:

  • Set BOARD_SUPER_PARTITION_BLOCK_DEVICES to the list of block devices used to store extents of dynamic partitions. This is the list of names of existing physical partitions on the device.
  • Set BOARD_SUPER_PARTITION_partition_DEVICE_SIZE to the sizes of each block device in BOARD_SUPER_PARTITION_BLOCK_DEVICES, respectively. This is the list of sizes of existing physical partitions on the device. This is usually BOARD_partitionIMAGE_PARTITION_SIZE in existing board configurations.
  • Unset existing BOARD_partitionIMAGE_PARTITION_SIZE for all partitions in BOARD_SUPER_PARTITION_BLOCK_DEVICES.
  • Set BOARD_SUPER_PARTITION_SIZE to the sum of BOARD_SUPER_PARTITION_partition_DEVICE_SIZE.
  • Set BOARD_SUPER_PARTITION_METADATA_DEVICE to the block device where dynamic partition metadata is stored. It must be one of BOARD_SUPER_PARTITION_BLOCK_DEVICES. Usually, this is set to system.
  • Set BOARD_SUPER_PARTITION_GROUPS, BOARD_group_SIZE, and BOARD_group_PARTITION_LIST, respectively. See Board configuration changes on new devices for details.

For example, if the device already has system and vendor partitions, and you want to convert them to dynamic partitions and add a new product partition during the update, set this board configuration:

BOARD_SUPER_PARTITION_BLOCK_DEVICES := system vendor
BOARD_SUPER_PARTITION_METADATA_DEVICE := system

# Rename BOARD_SYSTEMIMAGE_PARTITION_SIZE to BOARD_SUPER_PARTITION_SYSTEM_DEVICE_SIZE.
BOARD_SUPER_PARTITION_SYSTEM_DEVICE_SIZE := <size-in-bytes>

# Rename BOARD_VENDORIMAGE_PARTITION_SIZE to BOARD_SUPER_PARTITION_VENDOR_DEVICE_SIZE
BOARD_SUPER_PARTITION_VENDOR_DEVICE_SIZE := <size-in-bytes>

# This is BOARD_SUPER_PARTITION_SYSTEM_DEVICE_SIZE + BOARD_SUPER_PARTITION_VENDOR_DEVICE_SIZE
BOARD_SUPER_PARTITION_SIZE := <size-in-bytes>

# Configuration for dynamic partitions. For example:
BOARD_SUPER_PARTITION_GROUPS := group_foo
BOARD_GROUP_FOO_SIZE := <size-in-bytes>
BOARD_GROUP_FOO_PARTITION_LIST := system vendor product

SELinux changes

The super partition block devices must be marked with the attribute super_block_device_type. For example, if the device already has system and vendor partitions, you want to use them as block devices to store extents of dynamic partitions, and their by-name symlinks are marked as system_block_device:

/dev/block/platform/soc/10000\.ufshc/by-name/system   u:object_r:system_block_device:s0
/dev/block/platform/soc/10000\.ufshc/by-name/vendor   u:object_r:system_block_device:s0

Then, add the following line to device.te:

typeattribute system_block_device super_block_device_type;

For other configurations, see Implementing dynamic partitions on new devices.

For more information about retrofit updates, see OTA for A/B Devices without Dynamic Partitions.

Factory images

For a device launching with dynamic partitions support, avoid using userspace fastboot to flash factory images, as booting to userspace is slower than other flashing methods.

To address this, make dist now builds an additional super.img image that can be flashed directly to the super partition. It automatically bundles the contents of logical partitions, meaning it contains system.img, vendor.img, and so on, in addition to the super partition metadata. This image can be flashed directly to the super partition without any additional tooling or using fastbootd. After the build, super.img is placed in ${ANDROID_PRODUCT_OUT}.

For A/B devices that launch with dynamic partitions, super.img contains images in the A slot. After flashing the super image directly, mark slot A as bootable before rebooting the device.

For retrofit devices, make dist builds a set of super_*.img images that can be flashed directly to corresponding physical partitions. For example, make dist builds super_system.img and super_vendor.img when BOARD_SUPER_PARTITION_BLOCK_DEVICES is the system vendor. These images are placed in the OTA folder in target_files.zip.

Device mapper storage-device tuning

Dynamic partitioning accommodates a number of nondeterministic device-mapper objects. These may not all instantiate as expected, so you must track all mounts, and update the Android properties of all the associated partitions with their underlying storage devices.

A mechanism inside init tracks the mounts and asynchronously updates the Android properties. The amount of time this takes isn’t guaranteed to be within a specific period, so you must provide enough time for all on property triggers to react. The properties are dev.mnt.blk.<partition> where <partition> is root, system, data, or vendor, for example. Each property is associated with the base storage-device name, as shown in these examples:

taimen:/ % getprop | grep dev.mnt.blk
[dev.mnt.blk.data]: [sda]
[dev.mnt.blk.firmware]: [sde]
[dev.mnt.blk.metadata]: [sde]
[dev.mnt.blk.persist]: [sda]
[dev.mnt.blk.root]: [dm-0]
[dev.mnt.blk.vendor]: [dm-1]

blueline:/ $ getprop | grep dev.mnt.blk
[dev.mnt.blk.data]: [dm-4]
[dev.mnt.blk.metadata]: [sda]
[dev.mnt.blk.mnt.scratch]: [sda]
[dev.mnt.blk.mnt.vendor.persist]: [sdf]
[dev.mnt.blk.product]: [dm-2]
[dev.mnt.blk.root]: [dm-0]
[dev.mnt.blk.system_ext]: [dm-3]
[dev.mnt.blk.vendor]: [dm-1]
[dev.mnt.blk.vendor.firmware_mnt]: [sda]

The init.rc language allows the Android properties to be expanded as part of the rules, and storage devices can be tuned by the platform as needed with commands like these:

write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb 128
write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb 128

Once command processing starts in second-stage init, the epoll loop becomes active, and the values start to update. However, because property triggers aren't active until late-init, they can’t be used in the initial boot stages to handle root, system, or vendor. You may expect the kernel default read_ahead_kb to be sufficient until the init.rc scripts can override in early-fs (when various daemons and facilities start). Therefore, Google recommends that you use the on property feature, coupled with an init.rc-controlled property like sys.read_ahead_kb, to deal with timing the operations and to prevent race conditions, as in these examples:

on property:dev.mnt.blk.root=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.root}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on property:dev.mnt.blk.system=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.system}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on property:dev.mnt.blk.vendor=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.vendor}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on property:dev.mnt.blk.product=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.system_ext}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on property:dev.mnt.blk.oem=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.oem}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on property:dev.mnt.blk.data=* && property:sys.read_ahead_kb=*
    write /sys/block/${dev.mnt.blk.data}/queue/read_ahead_kb ${sys.read_ahead_kb:-2048}

on early-fs:
    setprop sys.read_ahead_kb ${ro.read_ahead_kb.boot:-2048}

on property:sys.boot_completed=1
   setprop sys.read_ahead_kb ${ro.read_ahead_kb.bootcomplete:-128}