Disclosed are systems, computer-readable mediums, and methods for efficiency sets in a distributed system. A first efficiency set is determined for a first volume of data. Determining the first efficiency set includes selecting block identifiers for data blocks of the first volume, where each block identifier is used to access a particular data block corresponding to the first volume. Determining the first efficiency set further includes applying a mask to the selected block identifiers to mask at least one bit of each selected block identifier. The first efficiency set is compared to a second efficiency set for a second data store, and based on the comparison, an amount of unique data blocks of the first volume is approximated.