The present invention relates to a method and an apparatus for data migration of a first file system in a data storage system comprising a plurality of client computers 100, a first file system managing unit 310 for managing the first file system and being connected to the plurality of client computers 100, and a second file system managing unit 410 for managing a second file system and being connected to the first file system managing unit 310, the second file system comprising a plurality of data files and the first file system comprising a plurality of external link objects, each external link object of the first file system being associated with a respective data file of the second file system for enabling client access to the respective data file of the second file system.