UBIFS
| Developer(s) | Nokia with help of University of Szeged |
|---|---|
| Full name | Unsorted Block Image File System |
| Introduced | 2008 with Linux kernel 2.6.27 |
| Structures | |
| Directory contents | B+ trees |
| Limits | |
| Allowed characters in filenames | Any byte except NUL and forward slash "/" |
| Features | |
| Forks | Yes |
| Attributes | Yes |
| File system permissions | POSIX |
| Other | |
| Supported operating systems | Linux |
UBIFS (UBI File System, more fully Unsorted Block Image File System) is a filesystem for unmanaged flash memory devices. UBIFS works on top of an UBI (unsorted block image) layer, which is itself on top of a memory technology device (MTD) layer. The file system is developed by Nokia engineers with help of the University of Szeged, Hungary. Development began in earnest in 2007, with the first stable release made to Linux kernel 2.6.27 in October 2008.
Two major differences between UBIFS and JFFS2 are that UBIFS supports write caching, and UBIFS errs on the pessimistic side of free space calculation. UBIFS tends to perform better than JFFS2 for large NAND FLASH devices. This is a consequence of the UBIFS design goals: faster mounting, quicker access to large files, and improved write speeds. UBIFS also preserves or improves upon JFFS2's on-the-fly compression, recoverability and power fail tolerance. UBIFS's on-the-fly data compression allows zlib (deflate algorithm) or LZO.
UBIFS stores indexes in flash whereas JFFS2 stores filesystem indexes in memory. This directly impacts the scalability of JFFS2 as the tables must be rebuilt every time the volume is mounted. Also, the JFFS2 tables may consume enough system RAM that some images may be unusable.
UBI (Unsorted Block Images) is an erase block management layer for flash memory devices. UBI serves two purposes, tracking NAND flash bad blocks and providing wear leveling. Wear leveling spreads the erases and writes across the entire flash device. UBI presents logical erase blocks to higher layers and maps these to physical erase blocks. UBI was written specifically for UBIFS so that UBIFS does not have to deal with wear leveling and bad blocks. However, UBI may also be useful with squashfs and NAND flash; squashfs is not aware of NAND flash bad blocks.
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