A cluster file system comprises a metadata server and a plurality of object storage servers each having one or more object storage targets. The object storage servers and their corresponding object storage targets are arranged in a first storage tier of the cluster file system. The cluster file system further comprises a second storage tier that includes a plurality of nodes of a scale-out network attached storage cluster. The cluster file system is configured to control movement of data between the first and second storage tiers. For example, in some embodiments, the object storage servers comprise one or more components of parallel log-structured file system (PLFS) middleware configured to control migration of data between the first and second storage tiers.