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Because data model of DAOS can support snapshot, overwrites, distributed transaction, MVCC…, so it is hard to change the metadata format. However, for workloads like AI/ML, the dataset for ingestion is will not not modified by AI/ML, It means that keeping indexes and logs for objects is not always necessary, especially for those objects are relatively small. The terminology to describe this kind of AI/ML datasets is Write-Once-Read-Many (WORM).
WORM object will never be modified again after completion of write, so it can be serialized, which is called “flattened” flattened in this design document, into contiguous buffer and migrate to DT-blob (unpinned from DRAM). After migrating to DT-blob, memory occupied by the object can be freed. In the future, before serving read, the entire object can be brought back to DRAM by one SSD read from DT-blob.
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