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Encyclopedia > Wear leveling

Wear levelling (also written -leveling) is a technique for prolonging the service life of some kinds of erasable computer storage media, such as flash memory. The term has also been used by Western Digital to describe their hard disk preservation technique, but hard disks are not generally wear-levelled devices. (The use of 'wear' is appropriate because it can refer to deterioration by use in addition to friction.) Service life refers to the expected lifetime of a product. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A USB Flash Memory Device. ... Western Digital Corporation NYSE: WDC (often abbreviated to WD) is a manufacturer of a large proportion of the worlds hard disks, and has a long history in the electronics industry as an IC maker and a storage products company. ... Typical hard drives of the mid-1990s. ... Typical hard drives of the mid-1990s. ...

EEPROM and flash memory media have individually erasable segments, each of which can be put through a finite number of erase cycles before possibly starting to wear-out and becoming statistically unreliable: anywhere between 10,000 and 1,000,000 cycles, for example, for NAND flash devices. 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). Often, each block or sector of data includes additional check bytes which can be used to detect errors and in many cases correct errors. There may also be a mechanism for remapping unreliable blocks to other unused blocks that are still working. Even before that point, the wear-levelling feature may detect that a certain block is getting changed very often, and swap it for another block elsewhere that has not yet been written to very much. An EEPROM (also called an E2PROM) or Electrically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. ... The optical lens of a compact disc drive. ... Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. ... The title given to this article is incorrect due to technical limitations. ... You can recognize a DVD-RAM immediately because visually there are lots of little rectangles distributed on the surface of the data carrier. ...

Conventional file systems like FAT, ext2 and NTFS were originally designed for magnetic disks and as such rewrite many of their data structures (such as their directories) repeatedly in place. This makes them poorly suited for use on media with erasure limitations. Wear-levelling 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 sector prematurely fails due to a high concentration of write cycles. In computing, a file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. ... For other uses, see Fat (disambiguation). ... The ext2 or second extended file system is a file system for the Linux kernel. ... NTFS or New Technology File System is the standard file system of Windows NT and its descendants: Windows 2000, Windows XP, Windows Server 2003 and Windows Vista. ...

SanDisk, for example, describes a technique whereby unused media sectors are grouped into blocks.[1] Each unused block is held in an erase pool until it is needed, and as additional writable space is requested, blocks are removed from the pool. Since sectors are grouped into blocks, a sector update can be performed by writing the updated data to a new sector within the block; the process is repeated until the block is filled. Once filled, a subsequent sector update would cause the sector to be written to an entirely new block, and the prior block is then erased and returned to the erase pool. This technique, however, is not used exactly as described in any of SanDisk's current products; it also may not be correct since it misses the fact that the FAT structure (on FAT-formatted flash drives) is frequently re-written in-place and cannot be reallocated or moved after wear failure, except possibly by grouping sectors that do and do not contain the FAT as part of the same block. SanDisk Corporation (NASDAQ: SNDK), formerly SunDisk, is a US-based multi-national corporation which designs and markets flash memory card products. ...

On flash memory devices, wear-levelling can be implemented in hardware by a built-in microcontroller or in software by special-purpose file systems such as JFFS or JFFS2. On optical media, the UDF file system provides wear-levelling. A microcontroller (or MCU) is a computer-on-a-chip used to control electronic devices. ... The Journaled Flash File System (or JFFS) is a log-structured file system for use on NOR flash memory devices. ... ... For other meanings of UDF, see UDF. The Universal Disk Format (UDF) is a format specification of a file system for storing files on optical media. ...

Some storage interfaces like DiskOnChip, Intelligent Stick or OneNAND (which allow flash memory devices to emulate regular ATA disks) do not in themselves perform wear levelling, but may do so with additional software below the filesystem layer. An emulator reproducing a console games playable atmosphere on a Windows computer. ... Typical hard drives of the mid-1990s. ...


  1. ^ SanDisk Flash Memory Cards Wear Leveling White Paper

External links

  • TrueFFS(R) Wear-Leveling Mechanism – By Dmitry Shmidt (May 20, 2002)
    (M-Systems Technical Note TN-DOC-017 Rev. 1.1 91-SR-002-55-7L)
  • Flash SSD wear-leveling and error-correction description by BiTMICRO.

  Results from FactBites:
TrueCrypt Forums :: Implications of wear levelling for volumes on flash drives (1570 words)
"Working at a slightly higher level than the device itself are various filesystem-level wear-levelling techniques which are used to decrease the number of erase operations which are necessary to update data, and the number of writes to a single segment of flash [[69]].
Because of this type of operation it’s not possible to cycle fresh cells to reduce remanence effects without bypassing the filesystem, in fact the operation of the wear-levelling system acts to create a worst-case situation in which data is always written to fresh cells.
This leads to the fact that writes are already a read/modify/erase/write process *even before wear levelling would require it*.
Wear levelling - Wikipedia, the free encyclopedia (308 words)
Wear levelling (also written -levelling) is a technique for prolonging the service life of some kinds of eraseable computer storage media, e.g.
Wear levelling involves arranging data so that erasures can be rotated evenly across the medium.
On flash memory devices, this can be performed in hardware by a built-in microcontroller or in software by a special-purpose file systems such as JFFS or JFFS2.
  More results at FactBites »



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