Solid State Drives

Solid State Drives (SSDs) go back in history a long ways.  Wikipedia has a nice historical summary and some great historical references.

The basic idea for SSD is to use semi-conductor or solid state memory for storage instead of rotating magnetic media.  The early super-computers from IBM, Amdahl, and Cray used custom SSDs for speed, albeit at great cost.  With improvements in semi-conductors following basically Moore’s Law, cost is coming down, capacity is improving, and the basic advantages of SSDs over hard disk drives (HHDs) remain.  Even better, some of the disadvantages of SSDs are being mitigated with more modern SSD and firmware technology.  My notes, accumulated for quite some time, cover these topics, and the plan here is to write them up in roughly the following order:

  1. NOR vs. NAND implementations.
  2. Multilevel vs. Single level cell (MLC vs. SLC) implementations
    …Does SLC really use less energy than MLC?
  3. Advantages of SSDs over HHDs
    …Low latency and fast access time
    …Zero “spin-up” time
    …Less power, less heat
    …Significantly less noise
    …Less susceptible to shock, vibration, temperature, and magnetic damage.
    …Less weight
    …Higher availability
    …Flexible form factors
  4. Disadvantages of SSDs over HHDs
    …Cost per megabyte
    …Limited number of writes
    …Cannot easily overwrite a block
    …Encryption performance poor
    …Generally poor write performance
    …Lower page corruption
    …Excess read disturbance
  5. Mitigation
    …DRAM write cache designs
    …Wear leveling: static vs. dynamic
    …Write amplification (=flash_write/host_writes)
    …Flash parallelism: multi-plane, multi-LUN
    …The TRIM function; block picking algorithms
    …Advanced (block or sector) error recovery
    …End to end data path protection
    …Life curve throttling
    …Minimizing power consumption
    …OS support
    …Standards (JEDEC, SATA, ONFI, PCIe, SNIA,…)
  6. Controller designs
  7. Host interfaces
  8. Special form factors
  9. Very large capacity SSDs
  10. Performance bench-marks
  11. The future of SSDs

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