This application concerns determining whether a particular subblock of data is present in a reduced-redundancy storage system. One embodiment achieves this by hashing subblocks in the storage system into a bitfilter that contains ‘1’ bit for each position to which at least one subblock hashes. This bitfilter provides a fast way to determine whether a subblock is in the storage system. In another embodiment, index entries for new subblocks may be buffered in a subblock index write buffer to convert a large number of random access read and write operations into a single sequential read and a single sequential write operation. The combination of the bitfilter and the write buffer yields a reduced-redundancy storage system that uses significantly less high speed random access memory than other systems that store the entire subblock index in memory.