1. A method of operating a cache sizing system, comprising: monitoring input/output (IO) accesses to a storage system within a monitor period; tracking an access map for storage addresses within the storage system during the monitor period; and counting a particular access condition of the IO accesses based on the access map during the monitor period, wherein, when sizing a cache of the storage system that enables the storage system to provide a specified level of service, the counting is for computing a working set size (WSS) estimate of the storage system.