Wear levelling

Wear leveling (also written as wear levelling) is a technique for prolonging the service life of some kinds of erasable computer storage media, such as flash memory, which is used in solid-state drives (SSDs) and USB flash drives, and phase change memory. There are several wear leveling mechanisms that provide varying levels of longevity enhancement in such memory systems.

The term preemptive wear leveling (PWL) has been used by Western Digital to describe their preservation technique used on hard disk drives (HDDs) designed for storing audio and video data. However, HDDs generally are not wear-leveled devices in the context of this article.

EEPROM and flash memory media have individually erasable segments, each of which can be put through a limited number of erase cycles before becoming unreliable. This is usually around 3,000/5,000 cycles but many flash devices have one block with a specially extended life of 100,000+ cycles that can be used by the Flash memory controller to track wear and movement of data across segments. Erasable optical media such as CD-RW and DVD-RW are rated at up to 1,000 cycles (100,000 cycles for DVD-RAM media).

Wear leveling attempts to work around these limitations by arranging data so that erasures and re-writes are distributed evenly across the medium. In this way, no single erase block prematurely fails due to a high concentration of write cycles. In flash memory, a single block on the chip is designed for longer life than the others so that the memory controller can store operational data with less chance of its corruption.

Conventional file systems such as FAT, UFS, HFS, ext2, and NTFS were originally designed for magnetic disks and as such rewrite many of their data structures (such as their directories) repeatedly to the same area. When these systems are used on flash memory media, this becomes a problem. The problem is aggravated by the fact that some file systems track last-access times, which can lead to file metadata being constantly rewritten in-place.

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