Wear Levelling and SSD Lifespan

Wear levelling is a process that extends the lifespan of solid-state drives and other flash storage devices, such as USB flash drives.

The NAND flash chips that make up solid-state drives (SSDs) and other devices are made up of blocks, and each block can withstand a certain number of read/write cycles before they wear out. Single-level cell (SLC) NAND flash can typically withstand between 50,000-100,000 program/erase cycles. Wear levelling is a process whereby data is arranged so write/erase cycles are distributed across all blocks equally. Assigned by the flash controller, there a several types of wear levelling that provide varying degrees of longevity in solid-state storage devices; the two main types are dynamic and static.

With dynamic wear levelling, the SSD takes erased blocks and selects the blocks with the lowest erase counts (i.e. the least used blocks) for the next write. While dynamic wear levelling is better than no wear levelling at all, with this method, wear levelling is limited to frequently modified, or hot areas. Blocks that hold “cold” data are not included in the pool of free space assigned by the flash controller. This limits the number of blocks that undergo wear levelling. A device that uses dynamic wear levelling might last longer than one that uses no wear levelling, but there will still be blocks that are active even if the device is inoperable.

Static wear levelling includes this cold, or static data in the wear levelling process, helping reduce wear in flash cells throughout the device. Static blocks containing cold data are periodically moved when their block erase count falls below a certain level. This method of wear levelling helps reduce wear of all cells across the device, rather than purely those containing frequently modified data.

Wear levelling is one of the best tools for extending the lifespan of a flash-based storage device, like an SSD or USB flash drive, by allowing program/erase cycles to be distributed evenly across all blocks in the device.

SSD Wear Levelling