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Reducing OTA Size

This page describes build changes added to AOSP to reduce unnecessary file changes between builds. Device implementers who maintain their own build system can use this information as a guide for reducing over-the-air (OTA) update size.

Android OTAs occasionally contain changed files that do not correspond to code changes but are instead artifacts of the build system. This can occur when the same code built at different times, from different directories, or on different machines produces a large number of changed files. These excess files not only increase the size of an OTA, but make it difficult to determine which code is changed in the OTA.

To make the contents of an OTA more transparent, AOSP includes build system changes designed to reduce OTA size by eliminating unnecessary file changes between builds. The aim is to reduce the size of OTAs to include only the files that relate to the patches contained in the OTA. AOSP also includes a build diff tool, which filters out common build-related file changes and provides a cleaner build file diff, and a block mapping tool, which helps you keep block allocation consistent.

The build system can create unnecessary file diffs in several ways. The following sections discuss some of these issues and solutions, providing examples of fixes in AOSP when possible.

File order

Problem: Filesystems don’t guarantee a file order when asked for a list of files in a directory, though it’s commonly the same for the same checkout. Tools such as ls sort the results by default, but the wildcard function used by commands such as find and make do not. Before using these tools, you must sort the outputs.

Solution: Users of tools such as find and make with wildcard must sort the output of these commands before using them. Use of $(wildcard) or $(shell find) in Android.mk files should also be sorted. Some tools, such as Java, do sort inputs so first verify sorting is necessary.

Examples: Many instances were fixed in the core build system using the builtin all-*-files-under macro, which includes all-cpp-files-under (as several definitions were spread out in other makefiles). For details, refer to the following CLs:

Build directory

Problem: Changing the directory in which things are built can cause the binaries to be different. Most paths in the Android build are relative paths so __FILE__ in C/C++ isn’t a problem. However, the debug symbols encode the full pathname by default, and the .note.gnu.build-id is generated from hashing the pre-stripped binary, so it will change if the debug symbols change.

Solution: AOSP now makes debug paths relative. For details, refer to CL: https://android.googlesource.com/platform/build/+/6a66a887baadc9eb3d0d60e26f748b8453e27a02.

Timestamps

Problem: Timestamps in the build output result in unnecessary file changes. This is likely to happen in the following locations:

  • __DATE__/__TIME__/__TIMESTAMP__ macros in C or C++ code.
  • Timestamps embedded in zip-based archives.

Solutions/Examples: To remove timestamps from the build output, use the instructions in the sections below.

__DATE__/__TIME__/__TIMESTAMP__ in C/C++

These macros always produce different outputs for different builds, so they shouldn’t be used. Here are a few options on how to eliminate these macros:

Embedded timestamps in archives (zip, jar)

Android 7.0 fixed the problem of embedded timestamps in zip archives by adding -X to all uses of the zip command, so the UID/GID of the builder and the extended Unix timestamp are not embedded in the zip file.

A new tool, ziptime (located in /platform/build/+/master/tools/ziptime/) resets the normal timestamps in the zip headers. For details, refer to the README file.

The signapk tool sets timestamps for the APK files that may vary depending on the server timezone. For details, refer to the CL https://android.googlesource.com/platform/build/+/6c41036bcf35fe39162b50d27533f0f3bfab3028.

Version strings

Problem: APK version strings often had the BUILD_NUMBER appended to the hardcoded version. Even if nothing else changed in the APK, the APK would still be different.

Solution: Remove the build number from the APK version string.

Examples:

Consistent build tools

Problem: Tools that generate installed files must be consistent (the same input should always produce the same output).

Solutions/Examples: Changes were required in the following build tools:

Using the build diff tool

For cases where it is not possible to eliminate build-related file changes, AOSP includes a build diff tool, target_files_diff.py for use in comparing two file packages. This tool performs a recursive diff between two builds, excluding common build-related file changes, such as:

  • Expected changes in the build output (for example, due to a build number change).
  • Changes due to known issues in the current build system.

To use the build diff tool, run the following command:

target_files_diff.py dir1 dir2

dir1 and dir2 are base directories that contain the extracted target files for each build.

Keeping block allocation consistent

In an non-A/B OTA, one of the factors that contribute to the time is block moves. For a given file, although its contents remain the same between two builds, the actual blocks that hold the data might have changed. As a result, the updater performs unnecessarily I/O to move the blocks around during an OTA.

To address this issue, in Android 7.0 we extended the make_ext4fs tool that tries to keep the block allocation consistent across builds. make_ext4fs accepts an optional -d base_fs flag that attempts to allocate files to the same blocks when generating an ext4 image. You can extract the block mapping files (i.e. the base_fs map files) from a previous build's target files zip file (IMAGES/system.map and IMAGES/vendor.map). The base_fs files can then be checked in and specified via PRODUCT_SYSTEM_BASE_FS_PATH and PRODUCT_VENDOR_BASE_FS_PATH. For example,

  PRODUCT_SYSTEM_BASE_FS_PATH := path/to/base_fs_files/base_system.map
  PRODUCT_VENDOR_BASE_FS_PATH := path/to/base_fs_files/base_vendor.map

While this doesn’t help reduce the overall OTA package size, it does improve OTA performance by reducing the amount of I/O.