Magnetic Disk Storage
surface consists of iron oxide or some other magnetizable oxide.
1.44MB Diskettes
The so-called "Floppy disks". invented in 1967 as
a "floppy" 5.25 inch (369 kilobyte) disk.
The rigid 3.5 inch diskette appeared around 1985.
Advantage: Small, inexpensive, movable, more or less reliable.
Disadvantage: small capacity and not fast enough.
What about data compression?
Other 31/2 inch "Diskettes"
Zip disk: 250 MB. much higher quality magnetic coating.
other types are also available, some can work with
ordinary floppy disks.
Hard Disks
Invented in the 50s.
A "pack" of disks in an airtight case
(to prevent dust and other particles
to go on the disks.
Both sides of each disks may be used.
Separate read/write head for each side of the disk.
Only one head may be active at any time.
Then it should come close to the disk surface but may NOT touch it
(otherwise "head crash")
Access arm contains all these read/write heads.
Organization of data on Hard Disks
- track: circular part of some disk
- sector: angular part.
When using zone recording,
you divide outer tracks in more sectors than inner tracks. ........
- cluster: fixed number of adjacent sectors on some track.
Each file reserves at least one sector.
- cylinder: corresponding tracks on ALL disks.
advantage to store data in cylinders: access arm doesn´t
have to move.
Disk Access Speed
- Seek time: time for the access arm to get in position.
- Head switching: Switch to the head you need, very small time.
- Rotational Delay: time to rotate the disk until the right part
is at the read/write head.
also important: data transfer rate (between 5 and 40 megabyte/sec)
that the drive can deliver to CPU.
average access time below 10msec now.
Improve performance by using disk cache:
"Read whatever is around, store it in cache, you may need it soon".
Magnetic Tape
May be a dying technology. Advantage: It is inexpensive.
Disadvantage: It is sequential, data cannot be accessed directly, but
you have to go through the whole tape when you want to get some
data.
Is still used for backup purposes.
RAID
redundant array of independent disks
- disk mirroring: all data is duplicated on another disk drive.
- data striping: distribute data on several disks, with one check disk.
in both methods, one should be able to recover data even if one disk crashes.
Optical Disk Storage
Writing: A laser "burns" tiny spots on the disk.
Reading: Laser scans the disk.
Main difference to magnetic storage devices:
Rewriting/erasing is more complicated.
Doable only with advanced material.
- ROM ("read only media")
- WORM ("write once, read many" times)
Compact Disks
CD-ROM ("compact disk---read only memory")
CD-R ("compact disk, recordable")(once only)
CD-RW ("compact disk, rewritable")
DVDs
"digital versatile disk", former "digital video disk".
two layers on one side, can be written on both sides,
uses laser with shorter wave length than CD.
DVD-ROM drive can also read CDs.
Magneto-Optical Disks
Combines the erasing/rewriting possibilities of magnetic disks
with high storage capacity of optical disks.
Laser makes part of surface "soft", magnet organizes
tiny magnetic crystals before material hardens.
Solid state Technology
no moving parts. Example: Flash memory, used for digital cameras.
chips with two-transistor cells.
How data is organized on disks
- character: letter, digit, or special character (one byte)
- field: collection of related characters,
key: a particular field allowing to identify the record
UNIQUELY.
- record: collection of related files,
- file: collection of related records,
- database: collection of interrelated files.
3 Methods to organize data
- sequential file organization: files stored in order (key field)
- direct file organization: requires disks, not tapes.
hashing, or randomizing creates an address from the key.
Whenever this is not unique (collision), the next available empty slot is used.
- indexed file organization: sequential organization, but with an key-address-index.
Processing stored data
- Batch processing: master file is updates periodically (every night, for instance)
transaction file must be sorted.
- Transaction processing: master file is updated immediately.
Real-time, online processing.
- Combinations of both ...
