Flash File System

Flash memory is widely used to hold a file system for an embedded device. The following is some facts about flash file system:

• Flash has two types: NAND and NOR flash. NOR is expensive and allows random-access for reading, NAND is cheap and allows only page access.
• Flash memory has finite number of erase/write cycles. >100K
• Flash devices allow programming values from a “1” to a “0”, but not from “0” to a “1” value. To program values back to “1”s requires erasing a full block. The time taken to erase blocks can be significant. In most cases when data is edited it must be written to a new location in flash and the old data invalidated. (Wear levelling)
• Flash file system types: JFFS2 and YAFFS.
• Flash file system contains of both code and data. There are two ways to execute the code: Store and Download (SnD) and eXecute in Place (XiP).

SnD (upper) and XiP (lower)

References:

• Flash Filesystems for Embedded Linux Systems 
• http://en.wikipedia.org/wiki/Flash_memory#Write_Endurance
• http://www.numonyx.com/Documents/WhitePapers/Flash_file_systems_WP.pdf
• http://en.wikipedia.org/wiki/File_system#Disk_file_systems