A data de-duplication in a distributed storage of data objects in a cluster system, in which plural data objects are distributed across a group of node apparatuses and stored in units of data blocks. Each metadata structure including a root metadata node and one or more direct metadata nodes, and optionally including one or more indirect metadata nodes; and a metadata object is stored for managing de-duplicated data blocks based on a metadata structure of the metadata object wherein at least one direct metadata node of the metadata structure of the metadata object includes a block reference pointing to a de-duplicated data block being associated with two or more data objects. Preferably, each of the metadata structures of the two or more data objects being associated with the de-duplicated data block includes a respective direct metadata node including an object reference to the metadata structure of the metadata object.