A method, system and computer program product for backup and restoration of data. Hash values for data blocks subject to backup are generated. After a number of hashes are accumulated, e.g., on a backup server, these hashes are sorted. Then, the hashes are compared against the hash values in the hash table corresponding to data blocks that have already been backed up. If a hash matches the hash from the hash table, a pointer to the block in the archive is written to the table of pointers to the redundant blocks. Then, this hash value is deleted from a set of the hash values. A check is made if a hash is the last in the group. If the hash is the last in the group, the remaining unique hash values are written into the hash table. Otherwise, the next hash is selected from the group. The redundant data blocks are discarded and only unique data is backed up.