1. A computer program product for extending a clustered file system, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to: define, by the processor, functions required by external services implemented on top of a clustered file system; from the defined functions, identify, by the processor, at least one function that is not provided by the clustered file system; and extend, by the processor, the clustered file system to provide the at least one function.