Wear leveling


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

Rationale

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.
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 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.

Types

There are three basic types of wear leveling mechanisms used in flash memory storage devices:

No wear leveling

A flash memory storage system with no wear leveling will not last very long if data is written to the flash. Without wear leveling, the underlying flash controller must permanently assign the logical addresses from the operating system to the physical addresses of the flash memory. This means that every write to a previously written block must first be read, erased, modified, and re-written to the same location. This approach is very time-consuming and frequently written locations will wear out quickly, while other locations will not be used at all. Once a few blocks reach their end of life, such a device becomes inoperable.

Dynamic wear leveling

The first type of wear leveling is called dynamic wear leveling and it uses a map to link logical block addresses from the OS to the physical flash memory. Each time the OS writes replacement data, the map is updated so the original physical block is marked as invalid data, and a new block is linked to that map entry. Each time a block of data is re-written to the flash memory, it is written to a new location. However, flash memory blocks that never get replacement data would sustain no additional wear, thus the name comes only from the dynamic data being recycled. Such a device may last longer than one with no wear leveling, but there are blocks still remaining as active even though the device is no longer operable.

Static wear leveling

The other type of wear leveling is called static wear leveling which also uses a map to link the LBA to physical memory addresses. Static wear leveling works the same as dynamic wear leveling except the static blocks that do not change are periodically moved so that these low usage cells are able to be used by other data. This rotational effect enables an SSD to continue to operate until most of the blocks are near their end of life.

Global wear leveling

Both dynamic and static wear leveling are implemented at the local level. This simply means that in a multi-chip product, every chip is managed as a single resource. The number of defective blocks in different chips within a NAND flash memory varies. A given chip can have all its data blocks worn out while another one can have all the blocks still active. To salvage this situation, global wear leveling is introduced. In global wear leveling, all blocks in all flash chips in the flash memory are managed together in a single pool. It ensures that all the cells in all the chips within the product are worn out evenly.

Comparison

The following table compares static and dynamic wear leveling:
ItemStaticDynamic
EnduranceLonger life expectancyShorter life expectancy
PerformanceSlowerFaster
Design complexityMore complexLess complex
Typical useSSDs,
industrial-grade flash drives
Consumer-grade flash drives

Techniques

There are several techniques for extending the media life:
On some specialist Secure Digital cards, techniques are implemented in hardware by a built-in microcontroller. On such devices, wear leveling is transparent, and most conventional file systems can be used on them as-is.
Wear leveling can also be implemented in software by special-purpose file systems such as JFFS2 and YAFFS on flash media or UDF on optical media. All three are log-structured file systems in that they treat their media as circular logs and write to them in sequential passes. File systems which implement copy-on-write strategies, such as ZFS, also implement a form of wear leveling.