1. An apparatus comprising: a physical file system comprising a plurality of storage tiers; a virtual file system configured to provide an interface between the physical file system and a plurality of clients of a computer system; and a metadata server associated with the virtual file system; the virtual file system comprising a plurality of virtual file system components implemented on respective ones of a plurality of compute nodes associated with the plurality of clients; wherein the virtual file system receives a plurality of files from each of one or more of the plurality of clients and arranges the received plurality of files into objects for object-based storage in the plurality of storage tiers of the physical file system; wherein the arranging of the files into objects comprises associating multiple ones of the received plurality of files with a single object of the virtual file system; wherein the virtual file system controls movement of the objects between the plurality of storage tiers of the physical file system; wherein the metadata server stores metadata information indicating, for each of the objects, a corresponding storage tier of the physical file system that currently stores the object; and wherein the metadata server comprises a network file server external to the virtual file system and each of at least a subset of the plurality of compute nodes incorporates a network file server mount configured to allow each compute node of the subset to access metadata stored on the network file server.