Data storage systems, methods and networks having a snapshot efficient block map

A data storage network comprising:

• one or more managers, each of which performs one or more storage management, file management and realm management tasks;
• and
• a plurality of data storage systems, each data storage system comprising a storage device, the storage device divided into a plurality of blocks for storing data for a plurality of volumes, and a processor to execute instructions for maintaining a block map corresponding to the data stored on the storage device, wherein the block map stores reference data indicating which of the volumes reference which blocks on the storage device, and which blocks on the storage device are unallocated, and wherein the reference data includes, for groups of one or more blocks, a pair of boundary values representing end points in an order of the volumes in which the group of blocks were allocated;
• and
• wherein the storage manager, the file manager, the realm manager, and the plurality of data storage systems include network communication equipment for communication through a network.