[SSD Basic] What is NAND flash memory, and what are its types?

NAND flash memory is the core storage technology in SSDs. It stores data in "cells," and different types of NAND are defined by how many bits of data each cell can store.

• SLC (Single-Level Cell): Stores 1 bit per cell.
  • Pros: Fastest, most durable (highest P/E cycles - Program/Erase cycles), highest data retention.
  • Cons: Most expensive, lowest capacity.
  • Usage: Primarily in high-end enterprise/server SSDs, or for caching within other SSDs.
• MLC (Multi-Level Cell): Stores 2 bits per cell.
  • Pros: Good balance of performance, endurance, and cost.
  • Cons: Less durable than SLC, more expensive than TLC.
  • Usage: High-end consumer SSDs, some enterprise applications.
• TLC (Triple-Level Cell): Stores 3 bits per cell. Also known as 3D NAND (vertical stacking of cells).
  • Pros: Lower cost, higher capacity.
  • Cons: Lower endurance than MLC/SLC, slightly slower.
  • Usage: Most common type in mainstream consumer SSDs (e.g., Samsung EVO series, Crucial MX series). Manufacturers use various technologies (like SLC caching, wear-leveling) to mitigate endurance concerns.
• QLC (Quad-Level Cell): Stores 4 bits per cell.
  • Pros: Lowest cost per GB, highest capacity.
  • Cons: Lowest endurance, generally slower (especially for sustained writes), lower data retention.
  • Usage: Value-oriented SSDs, for mass storage where writes are less frequent.
• PLC (Penta-Level Cell): Stores 5 bits per cell.
  • Emerging: Still largely in development or early adoption, pushing even higher densities. Expected to have even lower endurance and performance than QLC.

P/E Cycles (Program/Erase Cycles): Refers to how many times a NAND flash cell can be programmed (written) and erased before it degrades. SLC has the highest P/E cycles, QLC the lowest. Modern SSDs use sophisticated algorithms to manage wear and extend lifespan.