1. In a computing environment, a method performed at least in part on at least one processor, comprising: loading a subspace index comprising less than all index entries of a hash index service from a secondary media into a primary memory cache; and using the subspace index to deduplicate data, including chunking the data into one or more chunks, and for each chunk, determining whether a hash value computed for that chunk matches a hash value of an entry in the primary memory cache, and if not, storing the chunk and adding an entry for that hash value into the subspace index in association with a reference to that chunk, and if so, returning information by which the existing chunk is locatable.