stores one bit per cell, cells are floating gate MOSFETs
two types, NAND and NOR
NOR – used like traditional memory, execute-in-place memory (DRAM, SRAM)
NAND – accessed like a block device (disk), used for memory sticks, flash drived, MMC, CF
finite number of writes, unlimited reads
need wear leveling and bad block management
mostly NAND based, interface is smaller than, but electrically identical to, the ATA interface.
appears to the host device as if it were a hard disk. The CF device contains an ATA controller.
makes it easy to use CF to replace a small hard drive
used mostly in older digital cameras
convenient to use in ITX, mini-ITX systems that are IDE/ATA based
NAND technology developed by Siemans and SanDisk
been superseded by Secure Digital format
Secure Digital Card
most commonly found in digital camera equipment
typically formatted as FAT, FAT32 by manufacturer but can be reformatted to any file system (ext2, jffs, cram,yaffs) for embedded systems use
MMC card can be used in SD slot but not vice-versa
Journaling Flash File System
log-structured file system for use on NOR flash memory devices on the Linux operating system. It has been superseded by JFFS2
enforces wear leveling by treating the flash device as a circular log
At mount time, the file system driver must read the entire chain and then keep it in memory
The circular log design means all data in the filesystem is re-written, regardless of whether it is static or not. This generates many unnecessary erase cycles and reduces the life of the flash medium.
Journaling Flash File System (2)
includes support for NAND flash
better performance, JFFS treated the disk as a circular log. This generated a great deal of unnecessary I/O. The garbage collection algorithm in JFFS2 makes this mostly unnecessary.
supports hard links
part of Linux kernel since 2.4.10
Yet Another Flash File System
designed specifically for NAND flash cards
log structured, used both with embedded OSs and systems with no OS. Simple OS interface
Compressed File Systems
FS decompresses data as it is retrieved and may or may not compress as data is put into storage
read-only Linux file system
zlib-compressed one page at a time to allow random read access, files are compressed, meta-data is not
comes with a utility (mkcramfs) to pack files into new cramfs images.
often used for initrd images
set of patches for ext2 file system kernel driver to make it work with compression
not a new file system, makes ext2 work with both uncompressed and compressed data
meta data us left uncompressed (for safety)
doesn't require a separate partition for compressed files
Compressed read-only file system
uses gzip compression (LZMA being worked on)
Ubuntu, Fedora, Gentoo
often used with UnionFS to provide read/write environment for Live CDs
SLAX, Debian Live, Mandiva
allows several file systems to be mounted as a single file system
allows files and directories of separate file systems, to be transparently overlaid, forming a single coherent file system.
Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.
In the case of a union of a read-only and a writable FS where an identical path is encountered the preference can be given to the writable path.