Wear leveling in SSDs

In short: Flash memory cells endure only a limited number of write operations. Wear leveling spreads writes evenly so no cell fails prematurely.

In more detail: The SSD controller maps logical addresses to changing physical blocks. If the operating system keeps writing the same file, it lands somewhere else every time. All cells age at about the same speed.

In Depth

• Write cycles: From a few hundred to tens of thousands depending on cell type (SLC, MLC, TLC, QLC).
• TRIM: The operating system reports free blocks so the controller can erase them in advance and the SSD stays fast.
• Over-provisioning: An area reserved by the manufacturer eases distribution.
• TBW: The manufacturer figure “terabytes written” states the guaranteed write volume.
• Practice: Normal use rarely reaches the limit. SMART values show wear.

See also: SSDs