1. A computer program product comprising computer-readable program instructions which, when running on or loaded into a file system server or into a computer connected to a file system server or when being executed by one or more processors or processing units of the file system server or the computer, cause the file system server to execute a method for managing a file system in a data storage system comprising the file system server; the method comprising: managing a current version of the file system and a plurality of snapshot copies of the file system, each snapshot copy being a respective checkpoint version of the file system taken by a checkpoint mechanism at a respective checkpoint of a series of checkpoints, and executing a deduplication process for identifying and de-duplicating candidate data units for deduplication in the file system; the deduplication process including: taking a snapshot copy of the file system and processing said snapshot copy to identify data units, which have been modified since a past checkpoint up to a checkpoint of said snapshot copy, as candidate data units for deduplication, and processing a data unit identified from the identified candidate data units for deduplication to the current version of the file system, the identified data unit being identified from the identified candidate data units based on a condition which identified candidate data units have not been modified or excluded from their associated file system objects of the current version of the file system since the checkpoint of said snapshot copy.