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1 Disks Introduction ***-. 2 Disks: summary / overview / abstract The following gives an introduction to external memory for computers, focusing mainly.

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Presentation on theme: "1 Disks Introduction ***-. 2 Disks: summary / overview / abstract The following gives an introduction to external memory for computers, focusing mainly."— Presentation transcript:

1 1 Disks Introduction ***-

2 2 Disks: summary / overview / abstract The following gives an introduction to external memory for computers, focusing mainly on disks. This presentation is preceded by a more general introduction to computer technology. A separate but related presentation discusses optical disks like CDs and DVD. After an introduction, we focus on »Disk access time and data transfer rate »The capacity of various information carriers »Disk backup technologies **--

3 3 Magnetic disks Floppy disks Hard disks Disks: overview of various types Optical disks Laserdisc WORM CD-ROM DVD... ***-

4 4 !? Question !? Task !? Problem !? Which disk types do you know besides magnetic floppy disks and hard disks, and optical disks? **--

5 5 Disks: comparison of types Failure rate Disk capacity Storage cost per bit Speed of data access and transfer Exchangeability Transportability Risk of disk crashes Floppy High Low High Lowest ++ + Low Hard Low High - High Optical Low High Low + Low ***- Other High Low - +

6 6 Disks: decreasing prices: **-- Source cited in 1997: Source cited in 1997:

7 7 Disks: decreasing prices: 1995-… **--

8 8 !? Question !? Task !? Problem !? What are the consequences of the decreasing disk prices? **--

9 9 Disks: formats Physical format Logical format Applications formats Hardware Operating system Application programs **--

10 10 Physical and logical organization of disk files The logical view of the user is translated by the operating system to the physical level. User Operating system Logical view Physical view/structure/level **--

11 11 Data storage on disk: concentric tracks versus 1 spiral Storage method Concentric tracks One continuous spiral Access + Fast - Slow Usage Magnetic disks CDs **-- Capacity - Low + High

12 12 Constant Angular Velocity versus Constant Linear Velocity CAV CLV Disk rotation velocity Constant Changes as head moves along the spiral Usage Magnetic disks drives (and newer, faster CD players) CD players **--

13 13 Disks Disk access time and data transfer rate **--

14 14 Disk access time: definition Access time »= the time to locate a particular piece of information on a disk »= seek time + settling time + rotational delay **--

15 15 Disk access time: various causes Seek time = the time required to position the head assembly at the desired radius Settling time = the time required for the head to settle into position at the desired radius Rotational delay = the time the disk must rotate to get the desired sector under the read head **--

16 16 Disk access times of various types of disks Access time Hard disk from about 8 to 50 ms CD about 300 ms **--

17 17 Disks: data transfer rate Transfer rate Hard disk up to kB/s CD-ROM 150 (digital audio, 1x) up to kB/s (40x) Transfer rate = the speed at which the computer reads data from a disk once the data is found (kB/s) Floppy disk about 30 kB/s < **--

18 18 Disks Capacity of information carriers ***-

19 19 Capacity of information carriers Paper sheet A4 Floppy disks 5.25" ss-dd 5.25" ds-dd 5.25" ds-hd 3.5" ds-dd 3.5" ds-hd Hard disks on PCs Compact Disc 2 Kbytes 180 Kbytes 360 Kbytes Kbytes 720 Kbytes Kbytes from to Kbytes Kbytes ***-

20 20 Capacity of floppy disks: evolution ***-

21 21 Evolution of disk capacities: advantages and disadvantages Floppy disks + Hard disks Fast evolution: increasing capacity Incompatibilities between floppy disk and disk hardware Compact disc Stable from 1985: no increase of capacity High compatibility with CD and DVD drives ***-

22 22 Disks Disk backup technologies **--

23 23 Disk backup technologies: various systems Tape drives Removable drives Recordable CD (not rewritable) Recordable and rewritable CD Extra hard disk DVD-RAM Network drives **--

24 24 Disk backup technologies: tape drives +Pros: »inexpensive hardware »low media cost »large capacity -Cons: »slow »serial storage; no fast random access **--

25 25 Disk backup technologies: removable drives +Pros: »inexpensive hardware »random access storage -Cons: »limited to 2 GB (in 1999) **--

26 26 Disk backup technologies: recordable CD (not rewritable) Named CD-R The files are not erasable; not rewritable +Pros: »inexpensive; low media cost »random access storage »disks can be read by most CD-ROM drives -Cons: »limited to 680 MB »Not erasable / rewritable / reusable **--

27 27 Disk backup technologies: recordable and rewritable CD Named CD-RW The files are erasable; rewritable +Pros: »inexpensive; low media cost (but more expensive than CD-R) »random access storage -Cons: »limited to 680 MB »disks can NOT be read by most CD-ROM drives **--

28 28 Disk backup technologies: extra hard disk +Pros: »fast »random access storage -Cons: »expensive »not removable / cannot be kept off-site **--

29 29 Disk backup technologies: DVD-RAM +Pros: »random access storage »large capacity -Cons: »drives may be expensive **--


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