Secondary storage is much slower than memory (typical memory access = 10 ns). Disk access is slow, optical slower and tape even worse. For example, let's say we can move only one byte at a time:

• 1 MB memory = 5-10 ms to access and transfer
• 1 MB disk = 62 ms to access and transfer
• 1 MB 24X CD = 200 ms to access and transfer
• 1 MB tape = 60000 ms to access and transfer

Not all accesses cost the same: retrieving 1 MB that resides right next to the one you just read is usually much faster than getting one farther away. Let's see why...

MAGNETIC DISKS

[ HD, RAID ] [ Floppy, SuperDisk, Zip & Jaz ]

• each disk called PLATTER; usually all but top and bottom have 2 surfaces for storing info; info laid down in TRACKS (rings on surface); and each track subdivided into SECTORS; tracks directly above and below each other are CYLINDERS (all info on tracks of 1 cylinder are accessible w/o moving heads)
• moving the arm is the slowest part of the SEEK
• some disks have fixed heads, one per track - this is obviously more expensive, but much faster; these disks are typically not portable in any way
• disk capacity
  • depends on density of disk which depends on kind of disk
  • track capacity = sectors/track * bytes per sector
  • cylinder capacity = # tracks/cylinder * track capacity
  • drive capacity = # cylinders * cylinder capacity

physical placement of sectors; clusters; extents; fragmentation

• sectors often not continuous (rotation faster than data transfer rate) so skip (sometimes use blocks)
• clusters are sets of sectors that are continuous
• extents - file is all together in one piece
• fragmentation : file not all together in one piece
• blocks - user- or O/S defined on disk itself - instead of sectors; often more overhead; can be variable size (more overhead) or fixed (possible wasted space on a track) - can be quite efficient if set up right
• non-data overhead - more complicated schemes = more overhead; some overhead we cannot control

formatting disk involves initializing sectors/blocks; a place to keep addresses and condition (useable /defective); placing gaps and synchronization marks

seek time (~ 8ms)

• time for access arm to position the heads over the correct cylinder
• average time varies by use - sequential update will behave much differently from multi-file merge (typical = traverse 1/3 of #cylinders)

rotational delay (latency) (~ 4 ms)

• time taken for disk to rotate so that sector we want is under r/w head (Ave. = 1/2 revolution)

• getting around slowness of disks (rotation ~ 5200-7400 rpm)
  • striping (split file among several drives - allowing disks to transfer one after the other
  • use of RAM disk and disk cache as buffers

FLOPPY DISKS

• very old technology
• only 2 read/write heads; ~300 rpm, low density, poor quality materials (by today's standards),
• floppies have startup times and head-settle times in addition to seek, latency, & transfer which make them even slower

SUPERDISK (Imation [3M])

• fairly new: uses it's own drive which can also read/write regular floppies
• uses LS120 technology (optical tracks to align the heads)
• has capacities ~ 125 MB

ZIP & JAZ

• newer, better technology than floppies
• R/W heads smaller, records in zones rather than tracks (=same recording density throughout) ==> more sectors/track at outer edge
• much more dense, faster
• not as easily magnetized (bits take up less physical space)
• ZIP uses 1 disk, plastic, ~100 Meg
• JAZ uses 2 disks, metal, ~2 GB, more expensive (cost per byte is good)

RAID

• Redundant Array of Independent Disks
• used for increased performance; error recovery, and fault tolerance as well as large capacity
• disk controller is sophisticated (often CPU like x86), could implement your own RAID system
• data organized in various ways; from simple striping & mirroring to complicated interleaving patterns (by bit, block, or sector)

OPTICAL DEVICES

CD-ROM

• spin-
  off of audio CD technology
• non-magnetic --- non-volatile (more stable than magnetic disks)
• reflective surface on which are stamped pits; light shines down on surface and is reflected differently by pits than by rest of surface (the lands);
• takes >1 pit to 'register' a bit
• one byte on CD is actually 14 bits - translated by look-up table
• on 2X drive, pits fly past heads at rate of 1,200,000 pits/sec
• optical read/write heads are bigger and move slower than magnetic ones
• contain a single spiral track
• operate at a CONSTANT LINEAR VELOCITY - heads pass over the track at the same speed on outer edge as at inner edge - approx. 1000 rpm at the inner part of track and 400 rpm at outer edge on 2X drive
• maximize storage because all parts of track are recorded at maximum possible pit density
• spacing between tracks = 1.6 microns == > 15,000 tracks/inch
• access times 30-50 ms

MAGNETO-OPTIC

• 3.5" or 5.25"; 300 MB/side up to 2.5GB/side; double-sided
• drives with only one read head require that disks be turned over to read side 2
• single-sided 3 1/2" : 128MB- 640MB
• access time about 60 ms
• uses constant angular velocity (rpm) like magnetic disks
• re-writeable: gets heated to Curie Point (temp. at which molecules can be re-aligned by magnet); light is reflected differently depending on the polarity of the 'bit'
• use lower intensity light to read than write
• laser head to assist read can ride on a track rather than flying over the disk = less prone to crashes if it doesn't need to be close
• needs 2 passes to write (slow) but much more stable than other magnetic disks

WORMS

• write once; read many times
• has gold film on surface which gets scored to make 'pits'
• 5.25 - 12"
• capacity = 1.2 - 6.4GB

PD (Phase Change Optical Devices)

• uses laser to heat spots on the disk (making them less reflective); medium intensity reverts to original surface; lowest intensity is for reading
• high power; spiral track
• 650 MB capacity
• high durabilty - claims to withstand 500,000 rewrites

 

DVD

 

From Maxell's Web-Site:

DVD-RAM is also the first Phase Change media to employ Mark Edge recording. To increase pit density and boost recording capacity, both edges of a recording mark are used as signal positions. Maxell ensures that even with a many layered disc structure, by utilizing a phase change layer and optimizing thermal conductivity, the recording layer is highly sensitive to laser intensity, providing clear and precise recording marks.

FEATURES AND BENEFITS
High Capacity
5.2 Gigabyte (double-sided)

Reliable/Durable
As a result of many years of Optical Disk technology expertise, Maxell has acheived a highly reliable DVD-RAM disc with reduced noise, reduced disc tilt and enhanced durability of 100,000 read/write cycles.

Applications
Ideal for combined storage of music, video and data, transfer of video and large capacity data files, multimedia software title production, Internet downloads and massive archival storage.

SPECIFICATIONS
Model DVD-RAM5.2
Recording Capacity 5.2 GB
Laser Wavelength 650 nm
Minimum Data Bit Length 0.41µm
Sector Size 2,048 Bytes
Data Transfer Rate 11.08 Mbps
Cartridge Dimensions 4.88" x 5.25" x .3"

OROM

The right-hand picture shows the light from one organic LED shining up through one 32-KB data patch to the diffractive lens, which focuses the modulated light toward the reading mechanism. (Images courtesy of Ioptics, Inc.)

• no moving parts
• stationary organic LED's
• 128MB
• The data layer is arranged in an array of 5,000 "data
  patches.". A patch is actually a 2D image containing 32KBs of data. The data layer can be stamped into the plastic media for high-volume production needs. For lower-volume needs, data cards with a recordable layer will be made available for write-once recording.
• The unique illumination system for accessing data eliminates the need for moving parts, such as motors and moving arms.
• Data is stored and projected as large two-dimensional images onto a sensor, not read as a linear stream of single data bits.
• It uses organic light emitting diodes not a laser to read the data.
• Data access is 10 times faster than other optical devices
• Power consumption is less than 1 percent of the requirements for CD-ROM and DVD.
  

MEMORY-BASED DEVICES

PC Cards

• 3 types: I : 3.3 mm, II : 5 mm, III : 10.5mm
• low power consumption
• shock resistant
• light wieght
• very fast access
• Type I and II used for SRAM and Flash Cards
  • memory chip-based
  • very fast
  • SRAM is volatile but requires very little power
  • Type II often contains a modem
  • Flash doesn't need power but expensive (98: $50/Mb); capacity to 40 Mb
• Type III contains 1.8" HD

ROM / OTP Cards

• (OTP = one-time programmable)
• not used much
• too limited for the expense

TAPE DEVICES

OPEN REEL

• organization - tracks (often 9: 8 + parity)

1-bit length of tape is a frame

- gaps between blocks must be large enough to accommodate stopping and starting the tape

• length requirements - gaps plus length of blocks
• applications - great for sequential processing
  • - when direct access is infrequent
  • - archiving
  • - giant files

( about 30X cheaper than disk)

DATA LINEAR TAPE (DLT)

• 1/2" tape in cartridge
• developed by Digital
• single hub
• 10 - 35GB capacity
• fairly high speed
• 128 tracks in 64 pairs; some have 208 tracks
• takes ~ 90 sec to load; the tape is all fed into drive
• has directory at start of tape for faster searches

QIC TAPE (Quarter Inch Cartridge)

• older technology
• 125 MB uncompressed

DAT (Digital Audio Tape)

• also relatively old technology
• conforms to DDS (Digital Data Storage) standard
• helical tracks

PEREOS (teeny-weenie tapes)

• for teeny-weenie computers (?!)
• runs on 2 AA batteries
• connects to parallel port
• holds 1.25 GB
  

 

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CPSC 461: Copyright © 2002 Katrin Becker 1998-2002 Last Modified May 16, 2001 10:03 AM