The Journaling Flash File System (or JFFS) is a

log-structured file system A log-structured filesystem is a file system in which data and metadata are written sequentially to a circular buffer, called a log. The design was first proposed in 1988 by John K. Ousterhout and Fred Douglis and first implemented in 1992 by ...

for use on NOR

flash memory Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for t ...

devices on the

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

operating system. It has been superseded by JFFS2.

Design

Flash memory (specifically

NOR flash Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...

) must be erased prior to writing. The erase process has several limitations: * Erasing is very slow (typically 1–100 ms per erase block, which is 10³–10⁵ times slower than reading data from the same region). * It is only possible to erase flash in large segments (usually 64 KiB or more), whereas it can be read or written in smaller blocks (often 512 bytes). * Flash memory can only be erased a limited number of times (typically 10³–10⁶) before it becomes worn out. These constraints combine to produce a profound asymmetry between patterns of read and write access to flash memory. In contrast, magnetic hard disk drives offer nearly symmetrical read and write access: read speed and write speed are nearly identical (as both are constrained by the rate at which the disk spins), it is possible to both read and write small blocks or sectors (typically 512 or 4096 bytes), and there is no practical limit to the number of times magnetic media can be written and rewritten. Traditional file systems, such as

ext2 ext2, or second extended file system, is a file system for the Linux kernel (operating system), kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed ...

FAT In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers specif ...

which were designed for use on magnetic media typically update their data structures in-place, with data structures like inodes and directories updated on-disk after every modification. This concentrated lack of wear-levelling makes conventional file systems unsuitable for read-write use on flash devices. JFFS enforces wear levelling by treating the flash device as a circular log. All changes to files and directories are written to the tail of the log in ''nodes''. In each node, a header containing metadata is written first, followed by file data, if any. Nodes are chained together with offset pointers in the header. Nodes start out as ''valid'' and then become ''obsolete'' when a newer version of them is created. The free space remaining in the file system is the gap between the log's tail and its head. When this runs low, a garbage collector copies valid nodes from the head to the tail and skips obsolete ones, thus reclaiming space.

References

Sources

External links

JFFS Homepage (no longer maintained)

{{Filesystem Flash file systems supported by the Linux kernel Year of introduction missing

Design

See also

References

Sources

External links