Transcript Memory and File Systems

Memory
CS423
Dick Steflik
DRAM
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Dynamic Random Access Memory
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each data bit is stored in a separate capacitive
element in the Integrated Circuit
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Because capacitors leak their charge the memory
must be periodically refreshed, thus the name
Dynamic or Volatile
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requires one transistor and one capacitor per bit
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simple, allowing high packaging densities
refresh rate dependent on implementation
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8 usec – 64 msec
includes DDR and SDRAM
SRAM
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Static Random Access Memory
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Static in the sense that it does not need refreshing
like DRAM, but is still considered volatile (loses its
memory when power is removed)
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requires six transistors (multi ported memory may
require 8, 10 or more transistors per bit)
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lower packaging density than DRAM
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easier to interface with than DRAM because of
simplicity and no need to refresh
ROM
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Read-only memory
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mask ROM – non-volatile, permanently programed in the
manufacturing process
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PROM – Programmable Read-Only Memory
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essentially a fuse matrix in which programming is done by
blowing the fuses
EPROM – Erasable Programmable Read-Only Memory
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Programmable once then unalterable, non-volatile
semi-permanent in that the memory can be erased (by
exposing to ultrviolet light) and reprogrammed, considered to
be non-volatile
EEPROM – Electronically Erasable Programmable Read-only
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non-volatile, parallel and serial access.
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serial access via SPI, I2C, 1-wire
Flash
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Not an Acronym, just a name
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stores one bit per cell, cells are floating gate MOSFETs
two types, NAND and NOR
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NOR – used like traditional memory, execute-in-place
memory (DRAM, SRAM)
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NAND – accessed like a block device (disk), used for
memory sticks, flash drived, MMC, CF
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finite number of writes, unlimited reads
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need wear leveling and bad block management
CF
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Compact Flash
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mostly NAND based, interface is smaller than, but
electrically identical to, the ATA interface.
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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
MMC
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MultiMedia card
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NAND technology developed by Siemans and
SanDisk
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been superseded by Secure Digital format
SD, SDHC
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Secure Digital Card
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NAND Flash
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most commonly found in digital camera equipment
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typically formatted as FAT, FAT32 by manufacturer
but can be reformatted to any file system (ext2, jffs,
cram,yaffs) for embedded systems use
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MMC card can be used in SD slot but not viceversa
JFFS
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Journaling Flash File System
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log-structured file system for use on NOR flash
memory devices on the Linux operating system. It
has been superseded by JFFS2
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enforces wear leveling by treating the flash device
as a circular log
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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.
JFFS2
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Journaling Flash File System (2)
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includes support for NAND flash
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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.
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supports compression
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supports hard links
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part of Linux kernel since 2.4.10
YAFFS
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Yet Another Flash File System
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designed specifically for NAND flash cards
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log structured, used both with embedded OSs and
systems with no OS. Simple OS interface
Compressed File Systems
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FS decompresses data as it is retrieved and
may or may not compress as data is put into
storage
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CramFS
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e2compr
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SquashFS
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JFFS2
CramFS
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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
e2compr
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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
SquashFS
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Compressed read-only file system
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uses gzip compression (LZMA being worked on)
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Live CDs
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Ubuntu, Fedora, Gentoo
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often used with UnionFS to provide read/write
environment for Live CDs
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SLAX, Debian Live, Mandiva
UnionFS
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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.
Example: Knoppix Live CD with a USB memorystick