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  • persistent SCM: apache/barlow/crow pass (these are Optane / 3D XPoint produce code names), battery-backed DRAM or future SSD products supporting CXL.mem. This is the mode that DAOS supports today over PMDK. No changes will be done to this path.

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  • volatile SCM: DRAM or non-persistent CXL.mem memory. This mode requires an extra step on write operations to keep a copy of the metadata in sync on SSDs.


  • Phase 1: maintain up-to-date copy of metadata tree on SSDs

    • The SSD copy will be maintained up-to-date via a write ahead log (WAL) and regular background checkpointing.

    • All reads served from ram/tmpfs and metadata will be limited by the DRAM capacity

    • Extra work will be done to reduce metadata footprint

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  • Phase 2: metadata tiering

    • Metadata in dram/pmem is a subset of metadata on SSD

    • Potential cache-miss to SSD on read

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