This is a summary about a paper and a related talk in Sigmod 17, and this paper concludes the three main ways to make use of the special features in NVM when build up a DBMS.

NVM properties:

1. Byte addressable: Loads and stores un`like SSD/HDD
2. High random write throughput: Orders of magnitude higher than SSD/HDD, Smaller gap between sequential & random write throughput.
3. Read-write asymmetry & wear-leveling: Writes might take longer to complete compared to reads. Excessive writes to a single NVM cell can destroy it.

Evaluation NVM

=> HDD/SSD/Emulated NVM.
- Tune DRAM latency to emulate NVM (increasing latency)
- CLWB (Cache-line write back) instruction

Three levels in NVM DBMS:

1) Access Interfaces,
2) Storage Manager
3) Execution Engine.

Allocation interface

Standard functions
- Dynamic memory allocation
- Meta-data management
- Durability mechanism
  - Two-step implementation
    - Allocation first by using CLWB
    - Issues a SFENCE to make sure changes are visible
- Naming mechanism
  - Map NVM to a well know base address, Absolute pointer = Base address + Relative pointer
- Recovery mechanism
  - Consider a crashing before linking the chunk to DS which occur to losing of this chunk (neither allocator nor application can reclaim the space).
File system interface
- Device -> page cache -> app buffer => Device -> app buffer
- 64 bytes cache line atomic

Storage Manager

Logging & Recovery
- Write ahead log (LET’S TALK ABOUT STORAGE AND RECOVERY METHODS FOR NON-VOLATILE MEMORY DATABASE SYSTEMS SIGMOD' 2015)
  - Avoid duplicating data in the log and the checkpoint
    - logging the meta-data not the data updates
    - WAL + FS API vs. WA(meta-data)L + Allocation API
- Write behind log (WRITE-BEHIND LOGGING VLDB' 17)
  - pros
    1. Write-behind logging enables instant recovery
    2. Improves device utilization by reducing data duplication
    3. Extends the device lifetime
Data placement
- Three-tiers: DRAM (meta-data + data) + NVM (log + data) + SSD (data)
- DRAM -> hot data (cache), NVM -> warm data, SSD -> cold data

Access Methods

Read-write asymmetry & wear-leveling
– Writes might take longer to complete compared to reads
– Excessive writes to a single NVM cell can destroy it
Write-limited access methods
- NVM-aware B+Tree, hash table
  - NVM-aware B+Tree:
    - In leaf node, using linear scan (unsorted data) with fewer writes instead of binary search (sorted data) with more writes.
    - Using unbalanced tree (fewer writes)

Execution engine

Plan executor
- reduce the number of writes (Sorting algorithm and join algorithm) by using 1) adjustment of write-intensivity knob and 2) write-limited algorithms.
  - Hybrid join/sort
  - Segment sort => adjust x (x% merge sort + (1-x)% selection sort)