Erase Block Summary

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.

Features

JFFS2 introduced:
* Support for NAND flash devices. This involved a considerable amount of work as NAND devices have a sequential I/O interface and cannot be memory-mapped for reading.
* Hard links. This was not possible in JFFS because of limitations in the on-disk format.
* Compression. Four algorithms are available: zlib, rubin, rtime, and lzo.
* Better performance. JFFS treated the disk as a purely circular log. This generated a great deal of unnecessary I/O. The garbage collection algorithm in JFFS2 makes this mostly unnecessary.

Design

As with JFFS, changes to files and directories are "logged" to flash in ''nodes'', of which there are two types:
* '' inodes'': a header with file metadata, followed by a payload of file data (if any). Compressed payloads are limited to one page.
* ''dirent'' 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.
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.

Disadvantages

Due to its log-structured design, JFFS2's disadvantages include the following:

* All nodes must still be scanned at mount time. This is slow and is becoming an increasingly serious problem as flash devices scale upward into the terabyte range. To overcome this issue, the ''Erase Block Summary'' (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.
* Writing many small blocks of data can even lead to negative compression rates, so it is essential for applications to use large write buffers.
* 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.

See also

* List of file systems
* ZFS
* Btrfs
* NILFS
* F2FS

External links

JFFS2 websiteJFFS: The Journalling Flash File System
by David Woodhouse
PDF

JFFS2 official mailing list

References

{{Filesystem, state=collapsed

Disk file systems
Embedded Linux
Flash file systems supported by the Linux kernel
Compression file systems
Computer-related introductions in 2001