1. A method of maintaining a signature repository accessible by multiple client computing devices in a data storage system, the method comprising: tracking storage of a plurality of data units in a primary storage subsystem, the plurality of tracked data units corresponding to primary data generated by one or more applications executing on a plurality of client computing devices that form the primary storage subsystem, each data unit of the plurality of tracked data units forming at least a portion of at least one file stored in the primary storage subsystem, the primary data for each of the client computing devices stored in a primary data store associated with a respective client computing device, the primary storage subsystem in communication with a secondary storage subsystem that is separate from the primary storage subsystem and is configured to maintain secondary copies of at least some of the primary data; generating, by a signature agent executing on one or more processors in the primary storage subsystem, signatures corresponding to the plurality of tracked data units; and maintaining a signature repository including a signature block for at least each unique signature of the generated signatures, where each signature block comprises: the unique signature; and one or more data unit entries, each entry corresponding to a distinct data unit of the plurality of tracked data units and associated with the unique signature that is stored in the primary storage subsystem and that is generated by an application of the applications executing on a distinct client computing device, wherein each entry identifies a client computing device of the plurality of client computing devices that stores the corresponding distinct data unit, wherein at least one of the signature blocks includes at least a first entry indicating that a first primary data store associated with a first client computing device of the plurality of client computing devices stores a first data unit and that is associated with the unique signature and a second entry indicating that a second primary data store associated with a second client computing device of the plurality of client computing devices stores a second data unit that is associated with the unique signature, wherein the first data unit forms at least a portion of a first file stored in the first primary data store and the second data unit forms at least a portion of a second file stored in the second primary data store.