Wear Leveling - Techniques

Techniques

There are several techniques for extending the life of the media:

• A checksum or error-correcting code can be kept for block or sector in order to detect errors or correct errors.
• A pool of reserve space can also be kept. When a block or sector does fail, future reads and writes to it can be redirected to a replacement in that pool.
• Blocks or sectors on the media can be tracked in a least recently used queue of some sort. The data structures for the queue itself must either be stored off-device or in such a way that the space it uses is itself wear-leveled or, in the case of flash memory, in a block with a specially extended life.

M-Systems' product DiskOnChip used a software driver named TrueFFS which performed wear-leveling of NAND flash chips and made them appear to OS like a conventional disk drive. On most contemporary flash memory devices, such as CompactFlash and Secure Digital cards, these 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 as-is on them.

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.