A computer-implemented data backup method comprises storing metadata associated with an object in a secondary storage layer of a backup system, the metadata representing a data structure for the object, dividing a set of data representing the object into multiple variable length chunks, determining whether an existing chunk of data that is the same as at least a part of the variable length chunk exists in a primary storage layer of the backup system, storing the variable length chunk in the primary storage layer of the backup system, and on the basis of the determination, providing the primary storage layer of the backup system with an indication of the location of the existing chunk of data.