A method, system and computer program product for data backup such that: for each block of a storage device to be backed up to an image, generating a hash function value corresponding to contents of that block; generating a map of links between blocks in the image and corresponding blocks the storage device; using the hash function values to identify blocks of the storage device with identical contents, such that links for the blocks in the storage device with identical contents point to a single block in the image; and modifying the link in the map when a block in the storage is moved (for example, due to defragmentation) but its contents is not altered, so that the link points to the same backed up block.