1. A method for maintaining a mapping structure for maintaining metadata for snapshots in a virtualized storage environment, comprising: taking a snapshot of a virtual disk; generating an entry in a metadata structure for the snapshot, wherein the entry includes metadata for blocks in the snapshot that have been modified since a preceding snapshot, wherein the metadata comprise information indicating locations of physical data associated with the blocks in the snapshot; and lazily generating an entry in the mapping structure for the snapshot, wherein the entry includes values for each block in the snapshot, wherein a value for a block indicates a presence of metadata in the metadata structure for the block or an absence of metadata in the metadata structure for the block, the value for the block is either an on-bit or an off-bit, wherein a background process lazily generates the entry in the mapping structure for the snapshot at a later time after taking the snapshot.