Today

  • Flash-based SSD
    • Solid State => Circuitry
    • persistent storage
    • basic characteristics
    • from “row” flash => storage device

Basics

  • NAND Flash
  • “trap” charge, encode bit(s)
  • Bank/Chip
  • operations
    • read (page) (~2KB, 4KB)
    • ---------------------------------
|  |page|      |                |
---------------------------------
|     block    |
    • erase (block) (~256KB)
    • program (page)
    • to write
      • erase entire block
      • then, you can program each page within block exactly once
    • if you want to overwrite, must erase/prog again
  • Performance
    • read 10s of microseconds
      • much faster then hard drive
    • erase a few milliseconds
    • program 100s of microseconds
  • Reliability
    • “wear out”
      • once you erase/program block “too many” times
      • it stops working
      • “to many”: 10k ~ 100k times

SSD: Block-level Device

  • API: read, write block
  • Map to read, erase/program
  • Parallel storage device
|------------------------
| ------------  ------  |
| |Controller|  |DRAM|  |
| ------------  ------  |
| ---  ---              |
| |F|  |F|  ...         |
| ---  ---              |
-------------------------

Naive Approach Direct-mapped

  • to read:
    • read (easy)
  • to update a block:
    • read block
    • erase
    • program it again
  • Issue
    • I/O amplification (bad performance)
    • crash could lose data
    • wear things out

Log-structuring

  • treat flash blocks like a log
  •  Block1
--------------------------------------------------
|      |      |      |      |      |      |      |
--------------------------------------------------
  • How to find anything?
    • Flash Translation layer (FLT)
      • Implemented in controller + DRAM
      • (Map page->location on flash)
    • Problems:
      • FTL recovery in case of crash
        • When writing make sure to record mapping info
      • Need to do garbage collection
        • background activity
      • Wear leveling
        • background activity
      • Stpace overhead