1. A distributed metadata storage system, comprising: a plurality of compute nodes, wherein at least one of said compute nodes comprises a file system client that generates one or more of a directory metadata request and a file metadata request; a plurality of directory metadata servers to serve only directory metadata in response to a given directory metadata request; a plurality of file metadata servers to serve only file metadata in response to a given file metadata request, wherein said plurality of directory metadata servers are distinct from said plurality of file metadata servers, and wherein said plurality of directory metadata servers and said plurality of file metadata servers comprise at least one processing device and are connected to said plurality of compute nodes using at least one network; and at least one shared persistent key-value metadata store for storing said directory metadata and said file metadata, wherein said at least one shared persistent key-value metadata store is accessed by said plurality of directory metadata servers and said plurality of file metadata servers.