Answering Questions about Wear-Leveling Algorithms in SSD Controllers
Because NAND Flash blocks in SSDs have a limited number of erase cycles, it’s important for the longevity of the storage to maximize the use of these cycles. Wear-leveling algorithms do just that. With wear leveling, the endurance in NAND memory is increased to make SSDs as reliable as possible. How does it work? Here are the answers to questions engineers and other operators frequently have.
What exactly is wear leveling?
Within NAND storage, electrons can get trapped in the insulating oxide layer in the floating gate, and the oxide can break down because of hot carrier injections. These actions prevent NAND Flash from having an infinite number of program and erase cycles. Instead, most SLC NAND Flash storage devices work for 70,000 cycles. However, as the storage approaches this maximum usage, the cells become more unreliable. Wear leveling addresses this issue. Through wear-leveling algorithms, data from program and erase operations are evenly distributed on available blocks in the Flash drive. This activity happens automatically in the background, so that the host system is not affected.
How does it work?
Different SSD controller designers use different algorithms to achieve wear leveling, including both dynamic and static wear-leveling operations. In some cases, host data written for a Logical Block Address, or LBA, is physically stored where the lowest amount of endurance cycles are used. Because host data written to an LBA is not stored in the physical NAND location, in this scenario, a controller must track the translation.
Another approach involves static data that doesn’t move, such as applications data. This data can be moved by controllers to other NAND locations, opening up available storage space in NAND memory.
Are wear-leveling algorithms sufficient for improving NAND memory endurance?
Wear-leveling algorithms are essential for increasing endurance in SSDs, but it isn’t the only solution in use. There are multiple strategies working together to improve NAND stability in SSDs, including garbage collection algorithms and TRIM commands.
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