With the dramatic increase in the scale of computing, the applications of network storage systems become wider, and the requirements for their I/O performance are also higher. Now with I/O heavy loaded, it becomes meaningful to cache data by using low-speed media in the I/O path between the client and network storage systems. In this paper, a cache system prototype D-Cache is designed and implemented based on disk media at block level for storage system. A two-level structure is adopted to manage disk cache and a corresponding cache management algorithm is provided for it at block level. The algorithm effectively solves the management problem of disk cache brought by the low-speed characteristic of disk media. By the use of bitmap, the cache management algorithm also eliminates the overhead of Copy on Write operations caused by write miss of disk cache. Experimental results show that the prototype can efficiently improve the overall performance for storage system with I/O heavy loaded.