World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0002046462
Reproduction Date:

Title: Jffs  
Author: World Heritage Encyclopedia
Language: English
Subject: Boot File System, Journaling file system, UBIFS, Advanced Disc Filing System, Dell Fluid File System
Publisher: World Heritage Encyclopedia


Developer Axis Communications
Full name Journaling Flash File System
Supported operating systems Linux

The Journaling Flash File System (or JFFS) is a log-structured file system for use on NOR flash memory devices on the Linux operating system. It has been superseded by JFFS2.


Flash memory (specifically NOR flash) 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 or FAT 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.


  • At mount time, the file system driver must read the entire inode chain and then keep it in memory. This can be very slow. Memory consumption of JFFS is also proportional to the number of files in the file system.
  • The circular log design means all data in the filesystem is re-written, regardless of whether it is static or not. This generates many unnecessary erase cycles and reduces the life of the flash medium.

See also


  • Woodhouse, David. JFFS2: The Journalling Flash File System, version 2.

External links

  • Flash Filesystems for Embedded Linux Systems
  • JFFS Homepage (no longer maintained)
  • JFFS developer mailing list
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.