Flash Storage

Types

NOR Flash

• Fast read
• Slow write
• very slow erase
• Durability issue : 특정 cycle 이상 쓰면 damaged

NAND Flash

• NOR보다 Denser, Cheaper
• slow read / write / erase
• Durability issue : 특정 cycle 이상 쓰면 damaged

Flash memory overview

• Single / Multiple bits per cell
• Page : minimum unit of read / write
• Block : minimum unit of erasing

Operations

Read

Write

• 1 -> 0 transition만 가능 (empty page : 싹다 1)
  

Erase

SLC vs MLC

• performance 차이
• reliability 차이

FTL : Flash Translation Layer

overview

User

• logical하게 접근

FTL

• logical -> physical mapping
• wear leveling
• address translation
• garbage collection

Flash

• page 단위로 쓰기
• block 단위로 erase

FTL Function 1. Flash address mapping

Type 1. per-page mapping

• flexible
• large table
• good space utilization
• mapping table size = page 수 * table entry size

Type 2. per-block mapping

• small table
• write-amplification issue (1번 write했는데 write가 여러번 발생)
• mapping table size = block 수 * table entry size

FTL Function 2. Erase before program

상황 : 한 block 내에서 A,B page가 modified되었을 때.

valid page는 다른 block으로 모조리 옮긴 뒤, 해당 block을 erase

FTL Function 3. Wear-leveling

block마다 erase횟수 세는 counter 존재 -> erase 수 비슷하게 맞춰준다