Delta Sync
Also known as:
Block-Level Sync
Delta Encoding
Differential Sync
Definition
Delta sync transfers only the parts of a file that changed instead of the whole file. The file is treated as a list of chunks, each identified by a content hash. When the file is edited, the client re-chunks it, compares the new chunk hashes against the previous ones, and uploads or downloads only the chunks whose hashes differ. Editing a few bytes of a large file then costs kilobytes instead of the full file size.
Key Takeaways
- Delta sync moves only changed chunks, so a small edit to a huge file transfers very little data.
- It relies on chunking plus content-addressable storage to tell changed chunks from unchanged ones.
- Fixed-size chunking breaks on byte insertions because every later boundary shifts; content-defined chunking fixes that.
- It pairs with deduplication: unchanged chunks are already stored, so only new ones are written.
How It Works
- Represent each file version as an ordered list of content-hashed chunks.
- On edit, re-chunk the file and compute the new list of chunk hashes.
- Diff the new hash list against the old one to find which chunks are new.
- Transfer only the new chunks, then commit the new version’s chunk list.
Where It Is Used
- Dropbox uses block-level delta sync so editing a large file uploads only changed blocks.
- The rsync algorithm popularized transferring file deltas using rolling checksums.
- Version control and backup tools store successive versions as deltas to save space.