System and method for efficient space accounting in a file system with snapshots

A system and method for implementing on-the-fly block ownership accounting of snapshots in a file system is provided. For each snapshot, including the current active file system, the number of blocks that the snapshot is the youngest and oldest owner of is computed and stored in non-volatile storage. As file system operations modify the file system and associated snapshots, the youngest and oldest owner counts are updated concurrently with the execution of the file system operations, thereby providing near instantaneous access to the count information for use by various file system commands.