NetApp
Storage : Volumes : Understanding volumes : Using deduplication to increase storage efficiency

Using deduplication to increase storage efficiency
Deduplication is an optional feature of Data ONTAP that can improve physical storage space by eliminating duplicate data blocks within a FlexVol volume.
Deduplication works at the block level on an active file system, and uses the WAFL block-sharing mechanism. Each block of data has a digital signature that is compared with all other signatures in a data volume. If an exact block match exists, the duplicate block is discarded and its disk space is reclaimed.
You can configure deduplication operations to run automatically or according to a schedule. You can deduplicate new and existing data, or only new data, on any FlexVol volume.
Deduplication removes data redundancies, as shown in the following illustration: