Venti

Venti is a network storage system that permanently stores data blocks. A 160-bit SHA-1 hash of the data (called score by Venti) acts as the address of the data. This enforces a write-once policy since no other data block can be found with the same address: the addresses of multiple writes of the same data are identical, so duplicate data is easily identified and the data block is stored only once. Data blocks cannot be removed, making it ideal for permanent or backup storage. Venti is typically used with Fossil to provide a file system with permanent snapshots.