1. A filter system for a data storage system having a storage device being accessible to a client interface, the filter system comprising: a filter controller interposed between the client interface and the storage device and operable to receive an incoming request having protocol information from the client interface and provide an outgoing request to the storage device, the filter controller being further operable to receive an incoming response from the storage device and provide an outgoing response having protocol information to the client interface; a plurality of filters coupled to the filter controller and being responsive to the incoming request and the outgoing response to modify information related to the incoming request and the outgoing response, under control of the filter controller; and the filter controller being operative to select a filter of the plurality of filters based on the protocol information and to pass the protocol information to the filter, whereby the filter is further responsive to the protocol information, and wherein the filter is further selected by evaluating a logical expression created from data in an I/O map and an event map, wherein the I/O map is configured to hold identifiers for the plurality of filters and I/O operations for which the plurality of filters are registered, and wherein the event map is configured to hold the identifiers for the plurality of filters and characteristics of the plurality of filters.