The data storage system includes one or more storage apparatuses and a file system server connected to the one or more storage apparatuses and to one or more host computers. Write operations are controlled by providing a free space object which stores a plurality of indicators, each indicator being associated with one of a plurality of storage units for storing data units in the one or more storage apparatuses and each indicator indicating whether the associated storage unit is free or used, executing a read-ahead examination which comprises examining a set of indicators of the free space object for identifying one or more used storage units associated with indicators of the set, in particular for observing a fragmentation status of the storage units associated with the indicators of the set, and executing an allocation examination which comprises examining one or more indicators.