Maintaining snapshot and active file system metadata in an on disk structure of a file system

An on-disk structure of a file system has the capability to maintain snapshot and file system metadata on a storage system. The on-disk structure arranges file system data sequentially on disk within regions, wherein each region comprises a predetermined amount of disk space represented by blocks. The snapshot and file system metadata is maintained within level 1 (L1) indirect blocks of the on-disk structure. Each L1 indirect block describes (i.e., represents) a corresponding region of the on-disk structure of the file system; in the case of an active region, e.g., an L1 indirect block represents an active file data portion of a large file. The L1 indirect block that references an active region also performs file block number-to-disk block number mapping for the region.