Aflash file system is afile system designed for storingfiles onflash memory–based storage devices. While flash file systems are closely related to file systems in general, they are optimized for the nature and characteristics of flash memory (such as to avoidwrite amplification), and for use in particularoperating systems.
While ablock device layer can emulate a disk drive so that a general-purpose file system can be used on a flash-based storage device, this is suboptimal for several reasons:
Log-structured file systems have all the desirable properties for a flash file system.[1] Such file systems includeJFFS2 andYAFFS.
Because of the particular characteristics of flash memory, it is best used with either acontroller to perform wear leveling anderror correction or specifically designed flashfile systems, which spread writes over the media and deal with the long erase times of NAND flash blocks. The basic concept behind flash file systems is: when the flash store is to be updated, the file system will write a new copy of the changed data over to a fresh block, remap the file pointers, then erase the old block later when it has time.
In practice, flash file systems are used only forMemory Technology Devices (MTDs), which are embedded flash memories that do not have a controller. Removable flashmemory cards andUSB flash drives have built-in controllers to manage MTD with dedicated algorithms,[2][3] like wear leveling, bad block recovery, power loss recovery, garbage collection anderror correction, so use of a flash file system has limited benefit. However, some flash file systems, such asAPFS andF2FS, can be used on FTL-based flash devices such asSSD andeUFS.
Flash-based memory devices are becoming more prevalent as the number of mobile devices is increasing, the cost per memory size decreases, and the capacity of flash memory chips increases.
The earliest flash file system, managing an array of flash as a freely writable disk, was TrueFFS byM-Systems of Israel, presented as a software product in PC-Card Expo atSanta Clara, California, in July 1992 and patented in 1993.[4]
One of the earliest flash file systems wasMicrosoft's FFS2, for use withMS-DOS, released in autumn 1992.[5] FFS2 was preceded by an earlier product, called "FFS", which however fell short of being a flash file system, managing a flash array aswrite once read many (WORM) space rather than as a freely writable disk.
Around 1994, thePCMCIA, an industry group, approved theFlash Translation Layer (FTL) specification, based on the design of M-Systems' TrueFFS. The specification was authored and jointly proposed by M-Systems andSCM Microsystems, who also provided the first working implementations of FTL. Endorsed by Intel,[6] FTL became a popular flash file system design in non-PCMCIA media as well.
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Overlayfs, Unionfs, and aufs are union filesystems, that allow multiple filesystems to be combined and presented to the user as a single tree. This allows the system designer to place parts of the operating system that are nominally read-only on different media to the normal read-write areas.OpenWrt is usually installed on raw flash chips without FTL. It uses overlayfs to combine a compressed read-onlySquashFS withJFFS2.
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