Introduction

Wear leveling is a crucial technique for addressing the inherent limitations of NAND flash memory. It manages the wear rate across the entire NAND flash device, ensuring that each memory block is utilized as evenly as possible. Effective wear leveling is essential for maximizing the lifespan and reliability of the flash device. This process involves actively monitoring and controlling the program/erase (P/E) cycles of individual blocks to prevent excessive wear on specific areas and to distribute usage uniformly across memory.

Flash Block Management

A NAND flash device can generally be divided into three main functional areas:

• User Data Blocks: Reserved for storing user data and representing the logical capacity of the device.
• Free Blocks: Allocated for wear leveling operations and bad block management.
• System Blocks: Used by the Flash Translation Layer (FTL) for essential system functions such as mapping tables, cache, and firmware operations.

To ensure balanced wear across the memory array and extend the lifespan of the device, SP Industrial products implement three types of wear leveling techniques: dynamic, static, and global. Each method is tailored to manage different access patterns and operational requirements.

• Dynamic wear leveling targets frequently updated (dynamic) data. It assigns these writes to free blocks with the lowest program/erase (P/E) cycles, thereby balancing usage among active areas of the flash. This method is relatively simple to implement and effective for managing short-term wear. However, it does not address wear in blocks that store rarely updated (static) data, making it less comprehensive for long-term endurance.
• Static wear leveling expands wear management by considering the entire NAND flash die, including static data blocks that are rarely rewritten. It periodically relocates static data to less-used free blocks, enabling better distribution of wear across the entire memory array. This technique enhances the "liquidity" of data placement, helping to eliminate wear bottlenecks and improve overall memory efficiency. As a result, static wear leveling plays a key role in maximizing device lifespan.
• Global wear leveling provides the most comprehensive approach by monitoring wear across all zones of the flash device. It divides the memory into multiple regions and tracks write frequency across them. If a host consistently writes to the same zone, that zone may wear out faster than others. Global wear leveling detects such imbalances and intervenes by redistributing write operations to underused areas. This ensures uniform wear distribution across the entire device, significantly enhancing durability and reliability.