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When an object is created, it is always hierarchical, but it can either be in MD-blob or in DT-blob (memory bucket), it can be flattened by flattening service late if it is a WORM object. If a hierarchical object is put in MD-blob, then it will be pinned in DRAM, otherwise it is loaded into DRAM on demand.
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If it still cannot fit into DRAM, as described in previous section, DAOS can flatten the object index tree and move object structures into DT-blob and only keep internal index in MD-blob and DRAM. However, flattening object index tree requires extra works like defragmentation after punching objects, so this is an optional feature.
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