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JFFS2
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<h2 id="features">Features</h2> <p>JFFS2 introduced: </p> <ul><li>Support for <a href="/facts/NAND_flash/Cn8JsStx">NAND flash</a> devices. This involved a considerable amount of work as NAND devices have a sequential I/O interface and cannot be memory-mapped for reading.</li> <li>Hard links. This was not possible in JFFS because of limitations in the on-disk format.</li> <li>Compression. Five algorithms are available: <a href="/facts/Zlib/rExIchaK">zlib</a>, rubin, lzma, rtime, and <a href="/facts/Lzo/hfS0lRrD">lzo</a>.</li> <li>Better performance. JFFS treated the disk as a purely circular log. This generated a great deal of unnecessary I/O. The <a href="/facts/Garbage_collection_(computer_science)/j2vlIu7g">garbage collection</a> <a href="/facts/Algorithm/fnl5NmRt">algorithm</a> in JFFS2 makes this mostly unnecessary.</li></ul> <h2 id="design">Design</h2> <p>As with JFFS, changes to files and directories are "logged" to flash in <i>nodes</i>, of which there are two types: </p> <ul><li><i><a href="/facts/Inode/Ej7JIzyP">inodes</a></i>: a header with file metadata, followed by a payload of file data (if any). Compressed payloads are limited to one page.</li> <li><i>dirent</i> nodes: directory entries each holding a name and an inode number. Hard links are represented as different names with the same inode number. The special inode number 0 represents an unlink.</li></ul> <p>As with JFFS, nodes start out as <i>valid</i> when they are created, and become <i>obsolete</i> when a newer version has been created elsewhere. </p><p>Unlike JFFS, however, there is no circular log. Instead, JFFS2 deals in <i>blocks</i>, a unit the same size as the erase segment of the flash medium. Blocks are filled, one at a time, with nodes from bottom up. A <i>clean</i> block is one that contains only <i>valid</i> nodes. A <i>dirty</i> block contains at least one <i>obsolete</i> node. A <i>free</i> block contains no nodes.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p><p>The <a href="/facts/Garbage_collection_(computer_science)/j2vlIu7g">garbage collector</a> runs in the background, turning <i>dirty</i> blocks into <i>free</i> blocks. It does this by copying <i>valid</i> nodes to a new block and skipping <i>obsolete</i> ones. That done, it erases the <i>dirty</i> block and tags it with a special marker designating it as a <i>free</i> block (to prevent confusion if power is lost during an erase operation).<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p><p>To make <a href="/facts/Wear_levelling/N6ot7lGA">wear-levelling</a> more even and prevent erasures from being too concentrated on mostly-static file systems, the garbage collector will occasionally also consume <i>clean</i> blocks.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> </p> <h2 id="disadvantages">Disadvantages</h2> <p>Due to its log-structured design, JFFS2's disadvantages include the following:<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <ul><li>All nodes must still be scanned at <a href="/facts/Mount_(computing)/INDgdDX9">mount</a> time. This is slow and is becoming an increasingly serious problem as flash devices scale upward into the <a href="/facts/Terabyte/BNHUp9Qq">terabyte</a> range. To overcome this issue, the <i>Erase Block Summary</i> (EBS) was introduced in version 2.6.15 of the Linux kernel. EBS is placed at the end of each block and updated upon each write to the block, summarizing the block's content; during mounts, EBS is read instead of scanning whole blocks.</li> <li>Writing many small blocks of data can even lead to negative compression rates, so it is essential for applications to use large write buffers.</li> <li>There is no practical way to tell how much usable free space is left on a device since this depends both on how well additional data can be compressed, and the writing sequence.</li></ul> <h2 id="see-also">See also</h2> <ul> <li>Linux portal</li></ul> <ul><li><a href="/facts/List_of_file_systems/GMUaz18t">List of file systems</a></li> <li><a href="/facts/UBIFS/WkQzBBrr">UBIFS</a></li> <li><a href="/facts/NILFS/M2169PUW">NILFS</a></li> <li><a href="/facts/F2FS/8LWmaGAq">F2FS</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://www.sourceware.org/jffs2/">JFFS2 website</a></li> <li><a href="https://www.sourceware.org/jffs2/jffs2-html/">JFFS: The Journalling Flash File System</a> by David Woodhouse (<a href="https://www.sourceware.org/jffs2/jffs2.pdf">PDF</a>)</li> <li><a href="http://lists.infradead.org/pipermail/linux-mtd/">JFFS2 official mailing list</a></li> <li><a href="http://www.linux-mtd.infradead.org/faq/jffs2.html">JFFS2 FAQ</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Memory Technology Device (MTD) Subsystem for Linux". www.linux-mtd.infradead.org. Retrieved 2021-05-15. <a href="http://www.linux-mtd.infradead.org/faq/jffs2.html#L_hdd_jffs2" target="_blank">http://www.linux-mtd.infradead.org/faq/jffs2.html#L_hdd_jffs2</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"The OpenWrt Flash Layout - OpenWrt Wiki". Wiki.openwrt.org. 18 January 2010. Retrieved 2014-03-04. <a href="http://wiki.openwrt.org/doc/techref/flash.layout" target="_blank">http://wiki.openwrt.org/doc/techref/flash.layout</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Software Profile: Journaling Flash File System, Version 2 (JFFS2)" (PDF). micron.com. 2011. Archived from the original (PDF) on 2014-03-07. Retrieved 2014-03-04. <a href="https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf" target="_blank">https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Software Profile: Journaling Flash File System, Version 2 (JFFS2)" (PDF). micron.com. 2011. Archived from the original (PDF) on 2014-03-07. Retrieved 2014-03-04. <a href="https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf" target="_blank">https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Software Profile: Journaling Flash File System, Version 2 (JFFS2)" (PDF). micron.com. 2011. Archived from the original (PDF) on 2014-03-07. Retrieved 2014-03-04. <a href="https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf" target="_blank">https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Software Profile: Journaling Flash File System, Version 2 (JFFS2)" (PDF). micron.com. 2011. Archived from the original (PDF) on 2014-03-07. Retrieved 2014-03-04. <a href="https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf" target="_blank">https://web.archive.org/web/20140307004309/http://www.micron.com/~/media/Documents/Products/Software%20Article/SWNL_jffs2_profile.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> </ol>