Delta sync (block-level)
Also known as: block-level sync, differential sync, incremental file sync
Delta sync (block-level sync) uploads only the parts of a file that changed instead of the whole file. When you edit a large document, the service sends just the modified blocks, which makes syncing big files far faster and lighter on bandwidth.
- Uploads only changed blocks, not whole files
- Saves bandwidth and time on large edits
- Less effective on compressed or encrypted files
How delta sync works
A cloud client splits each file into small blocks and tracks a fingerprint for every one. When you save a change, it compares the new blocks against the stored fingerprints and transfers only the blocks that differ, rather than re-uploading the entire file.
For a large file where you change a single page or a few values, this can mean sending a tiny fraction of the data. Services such as Dropbox and OneDrive use block-level sync to keep edits to big files quick.
Where it helps and its limits
Delta sync mainly saves time and bandwidth, not local disk space — the full file still lives on your device unless you also use online-only or selective-sync options. It shines for documents, design files, and databases you edit repeatedly.
It is less effective on already-compressed or encrypted files, where a small edit can change most of the data, forcing a near-full upload. Some apps fall back to whole-file transfer in those cases.