Flash file systems

Flash file systems


Main article: Flash file system
Because of the particular characteristics of flash memory, it is best used with either a controller to perform wear-levelling and error correction or specifically designed flash file systems, which spread writes over the media and deal with the long erase times of NOR 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 only used for "Memory Technology Devices" ("MTD"), which are embedded flash memories that do not have a controller. Removable flash memory cards and USB flash drives have built-in controllers to perform wear-levelling and error correction so use of a specific flash file system does not add any benefit. These removable flash memory devices use the FAT file system to allow universal compatibility with computers, cameras, PDAs and other portable devices with memory card slots or ports.

 

Capacity


Multiple chips are often arrayed to achieve higher capacities for use in consumer electronic devices such as multimedia players or GPS. The capacity of flash chips generally follows Moore's Law because they are manufactured with many of the same integrated circuits techniques and equipment.

Consumer flash drives typically have sizes measured in powers of two (e.g. 512MB, 8GB).[citation needed] This includes SSDs as hard drive replacements, even though traditional hard drives tend to use decimal units. Thus, a 64GB SSD is actually 64¡Á10243 bytes. In reality, most users will have slightly less capacity than this available, due to the space taken by filesystem metadata.

In 2005, Toshiba and SanDisk developed a NAND flash chip capable of storing 1GB of data using Multi-level Cell (MLC) technology, capable of storing 2 bits of data per cell. In September 2005, Samsung Electronics announced that it had developed the world¡¯s first 2GB chip.

In March 2006, Samsung announced flash hard drives with a capacity of 4GB, essentially the same order of magnitude as smaller laptop hard drives, and in September 2006, Samsung announced an 8GB chip produced using a 40 nanometer manufacturing process.

In January 2008 Sandisk announced availability of their 16 GB MicroSDHC and 32 GB SDHC Plus cards.

But there are still flash-chips manufactured with low capacities like 1MB, e.g., for BIOS-ROMs.

 

Transfer rates


Commonly advertised is the maximum read speed, NAND flash memory cards are much faster at reading than writing. As a chip gets worn out, its erase/program operations slow down considerably, requiring more retries and bad block remapping. Transferring multiple small files, smaller than the chip specific block size, could lead to much lower rate. Access latency has an influence on performance but is less of an issue than with their hard drive counterpart.

The speed is sometimes quoted in MB/s (megabytes per second), or as a multiple of that of a legacy single speed CD-ROM, such as 60x, 100x or 150x. Here 1x is equivalent to 150 kilobytes per second. For example, a 100x memory card gives 150KB x 100 = 15,000KB/s = 14.65MB/s.

Performance also depends on quality of memory controllers. Even when the only change to manufacturing is die shrink, lacking an appropriate controller can cause degraded speeds.

 

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