We are assuming that map tasks with lower map ids are almost always run before those with higher map ids on a single node. Due to failures, this is not a guarantee, and in those cases we will create more Shuffle Files than are strictly necessary in order to maintain monotonicity. The worst case scenario would be running maps in order of descending id, in which case this would result in essentially running without shuffle file consolidation, but with more in-memory metadata and indirection.