[Feature Request]: Will DRAM-CXL-SSD tiered storage be supported in Mooncake

[qiuweit7]

Describe your feature request

DRAM-CXL-SSD Tiered Storage Implementation

Feature Description

This submission aims to implement the DRAM-CXL-SSD tiered storage core capability for Mooncake Store: the design retains hot data in ultra-low-latency DRAM, places warm data in a shareable CXL memory pool, and writes cold data to high-capacity SSD.

We are currently planning to merge this functionality into the community codebase. This Issue is submitted to synchronize the changes and solicit community feedback.

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Core Changes Overview

1. Client Initialization with Multi-Protocol Support

Files:

• mooncake-store/src/real_client.cpp
• mooncake-store/src/client_service.cpp

Changes:

• Added DispatchProtocols logic during client creation to implement protocol-to-storage level mapping:
  • tcp / rdma → StorageLevel::RAM
  • cxl → StorageLevel::CXL
• Mount RAM Segment and CXL Segment separately according to storage levels.

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2. Mount Implementation

Files:

• mooncake-store/src/client_service.cpp
• mooncake-transfer-engine/src/transfer_engine_impl.cpp

Changes:

• Adjusted transfer_engine memory registration logic (registerLocalMemory) to perform memory registration with the appropriate transport layer based on protocol selection.

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3. Level Allocation Strategy

File: mooncake-store/include/allocation_strategy.h

Changes:

• Added LevelAllocationStrategy class for tiered storage scenarios
• Route allocation requests based on ReplicateConfig.preferred_storage_level:
  • StorageLevel::RAM → allocateRAM() method
  • StorageLevel::CXL → allocateCXL() method

// Select allocation method based on specified storage level
ReplicateConfig config;
config.preferred_storage_level = StorageLevel::CXL;  // or StorageLevel::RAM

if (config.preferred_storage_level == StorageLevel::RAM) {
    return allocateRAM(...);
} else if (config.preferred_storage_level == StorageLevel::CXL) {
    return allocateCXL(...);
}

4. Protocol-Matched Data Transfer

Files:

• mooncake-store/src/transfer_task.cpp
• mooncake-transfer-engine/src/multi_transport.cpp

Changes:

• Added protocol handling logic to the submitTransfer interface

• Select protocol based on replica's storage level:

  // Select protocol based on replica storage level
  std::string proto = level_protocols_[replica.get_storage_level()];
  
  submitTransfer(requests, proto);

• Modified selectTransport to check whether the target segment's protocol list contains the requested proto, thereby obtaining the corresponding transport.

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[stmatengss]

Good feature. Hope to see related PRs.