A method includes constructing a tree of directories including M levels. M is an integer greater than 1, the tree includes a set of N directories of level 1, and N is an integer greater than 1. For each level X from 1 to M−1, each one of the directories of level X includes a set of N directories of level X+1. Storing a first file having a first identifier includes applying a hash function to the first identifier to generate a first hash, selecting a destination directory from level M by traversing the tree in response to the first hash, and storing the first file into the destination directory.