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A full OTA update can send a complete target image; a delta update instead sends instructions for constructing that target from a particular version already on the device. bsdiff is one tool for creating such binary patches. It can reduce download size when the versions share useful structure, but the patch must match the installed base—and it may be larger than a compressed full image.
What bundle diffing means in an OTA update
Bundle diffing is a descriptive term for comparing an older binary bundle or firmware image with a target version, then producing a patch that an updater can apply to reconstruct the target. The word “bundle” does not identify a universal patch standard: implementations may use different encodings, compression, or I/O models.
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With classic bsdiff, the producer compares an old file and a new file to make a patch intended for bspatch. The patch is relative to that specific source file. A device with a different base version cannot safely assume that the patch will work.
How bsdiff builds and applies a patch
Rather than simply listing literal changed bytes, classic bsdiff searches for approximate matches between the old and target files. It describes how to rebuild the target using control instructions, difference data, and extra data. The patcher uses the instructions to refer to corresponding bytes in the old file, combine them with difference data, and insert extra data where needed.
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The standard command-line relationship is:
bsdiff oldfile newfile patchfile
bspatch oldfile newfile patchfileThe first command creates a patch from the old and new files; the second applies that patch to the matching old file to produce the new one. The producer and updater must agree on the patch format and the exact source and target versions.
Where the patch fits in an OTA pipeline
An OTA system does more than calculate or apply a binary diff. It selects a target release, packages update metadata and data, transfers them to devices, runs the requested operations, verifies the result, and controls the transition to the new version. A binary patch can be one operation inside that larger process.
Android’s payload documentation describes an update payload as “an opaque blob with the instructions to update to the new version.” Its payload metadata can describe operations such as applying a binary patch. That does not establish that every Android patch operation uses classic BSDIFF40 or that an arbitrary bsdiff output can be inserted into an Android payload; the producer and consumer formats must be checked for the relevant platform version and toolchain. Android 9 and later also selects the compression algorithm expected to give the best compression results for a patch, illustrating that compression choices are part of the overall update pipeline.
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In Android’s A/B update design, changes are applied to an unused system slot while the current system remains available. The process verifies partitions against expected hashes, runs any required post-install step, and marks the new slot active. If the new version does not become successful, the boot process can return to the old slot. These are Android A/B behaviors, not features guaranteed by every OTA system.
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When a delta saves bytes—and when it does not
A delta can reduce download volume when releases retain enough reusable structure, especially when changes are localized. But the relevant comparison is the patch against a compressed complete target, not against an uncompressed image. A patch can carry overhead, and substantial differences between versions can make a full compressed image smaller.
In a 2003 paper, Colin Percival reported an average 11.6-fold compression for bsdiff across 19 pairs of historical DEC UNIX Alpha executable binaries. The paper reported 13.0-fold average compression after excluding the Apache 1.2.4-to-1.3.0 case, whose versions shared less than half their source code. These are results for that historical corpus, not estimates of savings for current firmware or OTA releases. The paper also found a case in which none of the tested methods beat simply compressing the new binary.
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Patch size is only one part of the decision. Generation can be demanding, and devices may have limited RAM, temporary storage, CPU time, or flash-write capacity. For the implementation described in the Debian bsdiff manual, dated 2003, memory use is reported as 17 times the old file size, with an absolute minimum working set of 8 times that size. Those figures apply to that implementation, not to every modern or embedded variant.
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Android’s A/B documentation gives about 100 KiB of metadata temporary storage in its Android 8.0 streaming-update context. That platform-specific figure is not the storage requirement of bsdiff itself.
How to evaluate a delta-update design
- Compare actual transfer sizes: measure each candidate patch against the compressed complete target for representative release pairs.
- Budget generation resources: measure build or release-system time and peak memory with the chosen implementation.
- Budget device resources: account for RAM, temporary storage, CPU time, flash writes, and whether the updater can process data as a stream.
- Confirm compatibility: document the patch encoding, compression, required source version, target architecture, and updater behavior.
- Protect correctness: verify the reconstructed result and define handling for interrupted transfers, retries, and failed updates.
- Plan operations: account for source-version variants, payload metadata, staged rollouts, and monitoring.
The right choice depends on measured payload size and device constraints, as well as the reliability and operational complexity the update system can support.
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