1. A method, in a data processing system, for extending cache for a storage system in a storage area network, the method comprising: responsive to a server data processing system performing a write operation to write a first portion of data to an external storage system, storing a copy of the first portion of data in a non-volatile memory of an extended cache card and updating a directory bit map in the extended cache card, wherein the extended cache card comprises a host side interface through which the extended cache card connects directly to a bus of the server data processing system, a storage side interface through which the extended cache card connects to a storage area network, and the non-volatile memory, wherein the server data processing system connects to the external storage system via the storage area network, and wherein the directory bit map keeps a record of which servers having extended cache cards store particular portions of data; responsive to the server data processing system performing a read operation to read a second portion of data from the external storage system, determining whether the extended cache card stores an up-to-date copy of the second portion of data; and responsive to a determination that the extended cache card stores an up-to-date copy of the second portion of data, sending the copy of the second portion of data from the extended cache card to the server data processing system.