1. A method of detecting duplicative hierarchically arranged sets of files in a storage system, comprising: generating, for hierarchically arranged plural sets of files, respective collections of values computed based on files in corresponding sets of files; generating, for a further set of files that is an ancestor of at least one of the plural sets of files, a respective collection of values that is based on the collection of values computed for the at least one set; and identifying duplicative sets according to comparisons of the collections of values.