In some examples, a system receives, in a synchronization buffer stored in a memory, fingerprint index entries for respective data units received by the system, each fingerprint index entry of the fingerprint index entries comprising a fingerprint computed for a corresponding data unit, and a storage location indicator for the corresponding data unit. The system merges the fingerprint index entries in the synchronization buffer with fingerprint index entries of a persistent fingerprint index in a hash data structure stored in persistent storage, the hash data structure comprising a plurality of buckets.