A data structure used in memory-based file system, method and apparatus using thereof. The data structure comprising: a tree of the nodes comprising tree nodes and leaf nodes, each tree node points to at least one node, each leaf node is associated with a plurality of data unit elements each of which representing a data unit, wherein each data unit element is associated with two pointers, wherein at least one of the two pointers is capable of pointing to a data unit or to a data unit element; and a cyclic linked list of data unit elements representing identical clones of a data unit, wherein the cyclic linked list comprises a first element pointing directly to the data unit, wherein from each element in the cyclic linked list, the data unit can be reached in time complexity of O(1).