Abstract: A computer assisted method of organizing a data structure for managing a computer storage device that contains a plurality of blocks. The method comprises initializing entries of the data structure to indicate that the blocks are shared between an origin and one or more snapshots and receiving a first request to modify a first portion of the storage device. The method also includes identifying a first chunk that contains the first portion to be modified, wherein the first chunk includes at least one block and writing first metadata into an entry of the data structure to indicate that the first chunk is unshared. The method also includes allowing the first chunk to be modified. Similar systems and software products are also described.