Flattened object index tree

If the storage server does not have massive DRAM, storing billions of objects in MD-blob and DRAM can still be impossible even each of them only consumes 100 bytes. In order to relieve memory pressure, VOS may also migrate object metadata (vos_obj_df) to DT-blob. As show in the diagram, VOS can copy objects to a contiguous buffer and store them in DT-blob. It means that DAOS can load objects to DRAM only if application is accessing the dataset, and totally evict object metadata from DRAM if the dataset is not being accessed.