wear leveling
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2009-02-20
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Publication Number Wear_Leveling_AN Revision 01 Issue Date April 30, 2008
1. Abstract
Today's embedded systems use Flash devices in data storage applications that may emulate a Hard Drive
(HDD); these types of erase/write intensive applications have highlighted concerns about Flash product life
expectancy. Flash memory devices are limited to a finite number of Program Erase (PE) cycles. An
embedded system's product life expectancy is tied directly to the Flash use model and the number of PE
cycles guaranteed by the Flash device. Using proper Flash management tools, the Flash life expectancy can
be extended to the point where the erase/write cycle limit has no consequence in many write-intensive
applications. A common technique used for extending the life expectancy of the Flash memory is wear
leveling. It is good practice to properly evaluate a systems Flash use model and determine if a tool like wear
leveling can be used to extend Flash life cycles per to meet the system requirements.
This document will highlight the following:
What is wear leveling?
Advantages of using wear leveling.
Wear Leveling mechanism.
2. Wear Leveling
Flash memory devices have an endurance specification that defines its maximum number of PE cycles. Flash
endurance is a sector-level specification that defines the conditions and the number of erase operations that
can be performed on a given Flash sector. Spansion
®
offers two types of non-volatile Flash memory: single-
bit-per-cell floating-gate Flash and Spansion's proprietary two-bit-per-cell MirrorBit
®
Flash. These two
technologies are designed to typically withstand 1,000,000 (Floating Gate) and 100,000 (MirrorBit) PE cycles
without cell failure.
In cases where the standard Flash endurance does not meet the projected cumulative erasures for a given
sector, wear-leveling algorithms provide a means to extend its expected Flash life expectancy. Wear-leveling
algorithms arrange or store data in a manner so that sector erasures are distributed more evenly across the
Flash cell array. A host system using a Flash File System performs its reads and writes to logical-sector
addresses. A wear-leveling algorithm is typically part of a Flash File System that remaps logical-sector
addresses to different physical-sector addresses in the Flash array. The wear-level algorithm tracks the least
used sectors in the Flash to identify where to next write data. This process reduces premature single sector
failures resulting from high concentration of erase cycles.
The Spansion Application Note: “Practical Guide to Endurance and Data Retention” provides additional useful
information to better understand Flash Endurance and Data Retention.
2.1 How Wear Leveling Effects Flash Life Expectancy
Below are two example cases that highlight how a system (Flash) life expectancy is affected by the decision
whether to implement wear leveling. Consider a system using a File Allocation Table (FAT) based file system
where the FAT table is being stored in the same logic sector. The following illustrates the number of times a
FAT table (FAT16 with a 2K cluster size) is updated when writing a 4 MB file to a 1 Gbit NOR Flash
(S29GL01GP).
Wear Leveling
Application Note
By: Ken Perdue
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