Backups and Deduplication

For creating a backup, restic scans the source directory for all files, sub-directories and other entries. The data from each file is split into variable length Blobs cut at offsets defined by a sliding window of 64 bytes. The implementation uses Rabin Fingerprints for implementing this Content Defined Chunking (CDC). An irreducible polynomial is selected at random and saved in the file config when a repository is initialized, so that watermark attacks are much harder.

Files smaller than 512 KiB are not split, Blobs are of 512 KiB to 8 MiB in size. The implementation aims for 1 MiB Blob size on average.

For modified files, only modified Blobs have to be saved in a subsequent backup. This even works if bytes are inserted or removed at arbitrary positions within the file.