A flash file system is a designed for storing on flash 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 avoid write amplification), and for use in particular operating systems.
While a block 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:
Erasing blocks: flash memory blocks have to be explicitly erased before they can be written to. The time taken to erase blocks can be significant, thus it is beneficial to erase unused blocks while the device is idle.
Random access: general-purpose file systems are optimized to avoid disk seeks whenever possible, due to the high cost of seeking. Flash memory devices impose no seek latency.
Wear leveling: flash memory devices tend to wear out when a single block is repeatedly overwritten; flash file systems are designed to spread out writes evenly.
s have all the desirable properties for a flash file system. Such file systems include JFFS2 and YAFFS.
Because of the particular characteristics of flash memory, it is best used with either a controller to perform wear leveling and error correction or specifically designed flash s, 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 for Memory Technology Devices (MTDs), which are embedded flash memories that do not have a controller. Removable flash memory cards and USB flash drives have built-in controllers to manage MTD with dedicated algorithms, like wear leveling, bad block recovery, power loss recovery, garbage collection and error correction, so use of a flash file system has limited benefit.
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Write amplification (WA) is an undesirable phenomenon associated with flash memory and solid-state drives (SSDs) where the actual amount of information physically written to the storage media is a multiple of the logical amount intended to be written. Because flash memory must be erased before it can be rewritten, with much coarser granularity of the erase operation when compared to the write operation, the process to perform these operations results in moving (or rewriting) user data and metadata more than once.
CompactFlash (CF) is a flash memory mass storage device used mainly in portable electronic devices. The format was specified and the devices were first manufactured by SanDisk in 1994. CompactFlash became one of the most successful of the early memory card formats, surpassing Miniature Card and SmartMedia. Subsequent formats, such as MMC/SD, various Memory Stick formats, and xD-Picture Card offered stiff competition. Most of these cards are smaller than CompactFlash while offering comparable capacity and speed.
A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is also sometimes called a semiconductor storage device, a solid-state device or a solid-state disk, even though SSDs lack the physical spinning disks and movable read–write heads used in hard disk drives (HDDs) and floppy disks. SSD also has rich internal parallelism for data processing.
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