A computer-implemented method for indexing content stored in a cache memory device is disclosed. The method starts with maintaining a file index having a plurality of extent entries, each extent entry corresponding to one of a plurality of file extents stored in a cache memory device that caches data stored in a persistent storage device of a storage system. In response to receiving a request to read a first file region of a first file, the method continues with retrieving first data block from the persistent storage device that contains the first data block, caching the first data block at a first storage location of the cache memory device; and creating a first extent entry in the file index having at least a first node, where the first node includes an address of the first storage location and a first bitmap indicating which data blocks are valid.