JFFS2  Journalling Flash File System version 2 or JFFS2 is a log-structured file system for use with flash memory devices. It is the successor to JFFS. JFFS2 has been included into the Linux kernel since September 23, 2001, when it was merged into the Linux kernel mainline as part of the kernel version 2.4.10 release. JFFS2 is also available for a few bootloaders, like Das U-Boot, Open Firmware, the eCos RTOS, the RTEMS RTOS, and the RedBoot. Most prominent usage of the JFFS2 comes from OpenWrt. At least three file systems have been developed as JFFS2 replacements: LogFS, UBIFS, and YAFFS. JFFS2 introduced:  As with JFFS, changes to files and directories are "logged" to flash in "nodes", of which there are two types: As with JFFS, nodes start out as "valid" when they are created, and become "obsolete" when a newer version has been created elsewhere. Unlike JFFS, however, there is no circular log. Instead, JFFS2 deals in "blocks", 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 "clean" block is one that contains only "valid" nodes. A "dirty" block contains at least one "obsolete" node. A "free" block contains no nodes. The garbage collector runs in the background, turning "dirty" blocks into "free" blocks. It does this by copying "valid" nodes to a new block and skipping "obsolete" ones. That done, it erases the "dirty" block and tags it with a special marker designating it as a "free" block (to prevent confusion if power is lost during an erase operation). To make wear-levelling more even and prevent erasures from being too concentrated on mostly-static file systems, the garbage collector will occasionally also consume "clean" blocks. Due to its log-structured design, JFFS2's disadvantages include the following: