An ordered data object identifier denoted by a value is assigned to each data object grouped to a container object, wherein data object identifiers with successively incremented values are assigned to successive data objects. An ordered container identifier is assigned to each container object with the greatest value of the identifiers of the data objects grouped thereto. A metadata structure with an entry for each container objects is stored. Each entry comprises the ordered container identifier and a reference to the corresponding data. The metadata structure is ordered according to the values of the container identifiers. A request to read a data object contains a corresponding data object identifier. It is determined to which container object the data object is grouped, by locating the first container identifier in the metadata structure with a value greater than or equal to that the requested data object.