In one example, a method includes measuring an amount of physical storage space used, or expected to be used, by a portion of a dataset S of segments, and measuring the amount of physical storage space includes receiving information that identifies an ad-hoc group of size ‘n’ of files F1 . . . Fn that makes up a subset of the dataset S, determining a number of unique segments in the dataset S, identifying a respective unique segment set UF1 . . . UFN for each of the ‘n’ files in the ad-hoc group of files, performing a set union operation on the unique segment sets UF1 . . . UFN, and determining a sum of sizes of the unique segment sets UF1 . . . UFN, where the sum is the amount of physical storage space used or expected to be used by the ad-hoc group of size ‘n’ of files F1 . . . Fn.