Apparatus for data migration of a first file system in a data storage system comprising a plurality of client computers, a first file system managing unit for managing the first file system and being connected to the plurality of client computers, and a second file system managing unit for managing a second file system and being connected to the first file system managing unit, the second file system comprising a plurality of data files and the first file system comprising a plurality of virtualized files, wherein, for each of the data files existing in the second file system, a respective associated virtualized file exists in the first file system, each virtualized file in the first file system being an external link object, which exists in a directory of the first file system corresponding to a respective directory in the second file system having the associated data file, each external link object enabling indirect client access through the respective external link object to the respective associated data file in the second file system before actual user data of the respective file is migrated from the second file system to the first file system, and the external link objects of the first file system are presented as files to the clients; the apparatus comprising the first file system managing unit adapted to execute a file system migration process to migrate file content data for each of the virtualized files of the first file system from the respective associated data file existing in the second file system, wherein executing the file system migration process, for a first file of the second file system, includes: - dividing file content data of the first file of the second file system into a first group of migration data blocks, and - migrating file content data of the first file of the second file system from the second file system managing unit to the first file system managing unit in units of migration data blocks of the first group of migration data blocks, wherein client access to data content of a migration data block of the first group of migration data blocks is temporarily held off while the respective migration data block is migrated from the second file system managing unit to the first file system managing unit, and client access requests directed to the respective migration data block of the first group of migration data blocks received during migration of the respective migration data block are queued and executed after the respective migration data block of the first group of migration data blocks has been migrated, and wherein client access to data content of the first group of migration data blocks is enabled for all migration data blocks, which have been migrated from the second file system managing unit to the first file system managing unit, and for all migration data blocks, which have not yet started migration from the second file system managing unit to the first file system managing unit. - dividing file content data of the first file of the second file system into a first group of migration data blocks, and - migrating file content data of the first file of the second file system from the second file system managing unit to the first file system managing unit in units of migration data blocks of the first group of migration data blocks,