A classic filesystem associate each path with a content. A content-addressable filesystem associate each path to the hash of its content, and then keeps a sepa

A classic filesystem associate each path with a content. A content-addressable filesystem associate each path to the hash of its content, and then keeps a separate mapping of each hash to a content. Which means that if two files are identical (= have the same hash), then the content is only stored once. A union filesystem is a filesystem that is organised as a set of "layers". You have a normal filesystem at the bottom, above it you have a set of layers, where each layer only stores the files that have changed (each time you create a snapshot, the filesystem creates a new layer). In practical terms, you have a filesystem where taking snapshots and reverting to older snapshots is very efficient. Or, in other words, a filesystem that behaves like a git repository.