Understanding NAND Flash Memory in SSDs: Types, Challenges and Advancements
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It is important to understand the different types of NAND flash memory and their associated technologies because they play a critical role in modern flash solid-state drives (SSDs). Flash solid-state drives are semiconductor-based non-volatile storage that stores persistent data in flash memory. Two main flash memory types are used in SSDs, NAND, and NOR, with NAND being the most commonly used type in SSDs. NOR is generally used in code execution; for example, a smartphone might utilize an embedded NOR memory chip to boot up the OS and use NAND for storage.
NAND flash memory chips consist of an array of blocks and an array of memory cells within each block. NAND flash memory breaks down into several design types, including "Single Level Cells" (SLC), Multi-Level Cells (MLC), "Triple-Level Cells" (TLC), "Quad-Level Cells" (QLC), and "Penta-level Cells" (PCL), with the difference being the number of bits per memory cell.
Each added bit comes with a performance, endurance and reliability penalty. SLC offers the highest performance, endurance and reliability but has a high price per GB and is often used in commercial and industrial applications. MLC and TLC are both cheaper and denser and better options for write-intensive applications. QLC is primarily for read-intensive workloads such as data analytics and machine learning. PLC offers limited endurance and a low cost per GB and is suitable for archival applications.
NAND flash memory doesn't come without its share of challenges. Primarily flash memory has a finite number of program/erase (P/E) cycles. Several techniques are utilized to manage this limitation, including garbage collection, wear leveling, overprovisioning and TRIM, to increase SSD reliability and endurance. SSD endurance is a complex topic, with each vendor leveraging algorithms and optimization techniques specific to their application.
NAND capacity growth is achieved by shrinking cell sizes. Unfortunately, the cells must have enough space to store sufficient electrons. Too few electrons and cell function decrease and cell-to-cell interference increases, negatively impacting performance, endurance, and reliability.
3D NAND technology was developed to address the limitations of 2D planar technology and has since become the leading way to achieve NAND capacity growth. Unlike 2D planar technology, where the cells are stacked horizontally, 3D NAND is stacked vertically using multiple layers. These added layers increase density, lower power consumption, increase endurance and performance, and lower the overall cost per gigabyte. In addition to NAND and 3D NAND, it's worth mentioning 3D XPoint technology, which is a newer type of non-volatile memory that offers higher performance, lower latency, and better durability compared to NAND. However, it's currently more expensive and less dense than NAND, making it a niche option for select use cases that require extremely high performance.
NAND flash memory is at the heart of modern SSDs, with new 3D NAND technology making it possible to have ever-increasing capacity and performance. Picking the right flash memory for each application can be difficult. Each type has its own advantages and disadvantages but understanding the cost and performance benefits of each can be key to getting the best value for the application.