Splitting files into variable-sized blocks so small edits move kilobytes, not gigabytes.
Chunking is the process of splitting files into variable-sized blocks so a small edit doesn’t force the engine to re-upload an entire large file.
Naive backup tools treat the file as the unit: change one byte in a 40 GB Lightroom catalog, re-upload 40 GB. Chunking uses a rolling hash — content-defined boundaries — to slice that catalog into thousands of roughly 4 MB pieces, each hashed and stored independently. Edit a single image in the catalog and typically two or three chunks change. The rest are already in the repository and get referenced by hash. The next snapshot uploads a few megabytes, not forty gigabytes.
The “content-defined” part matters. Fixed-size chunking (every 4 MB starting from byte zero) would break the moment you insert or delete bytes near the front of a file — everything downstream shifts, and every chunk looks new to the engine. Rolling-hash boundaries move with the content, so insertions and deletions only affect the chunks around them. This is what makes dedup work on real-world files like video timelines, database dumps, and virtual-machine images.
In macup, chunking runs on your Mac before upload. The engine slices files, hashes each chunk, encrypts the chunks with your key, and only then uploads. The destination — macup Cloud, an external SSD, or an S3 bucket — stores encrypted, content-addressed blocks rather than whole readable files.