1. Secondary storage Devices

2. Primary storage
The primary storage of a computer system has the following limitations:
Limited Capacity:
It is often necessary to store many millions, sometimes billions, and even trillions, of bytes of data in a computer.
The storage capacity of the primary storage of today’s computers is not sufficient to store the large volume of data, which needs to be handle by most data processing centers.
Volatile :
The primary storage is volatile, and the data stored in it is lost, when the electric power is turned off or interrupted.
Computer systems need to store data on a permanent basis for several days, several months, or even several years.

3. Secondary storage
As a result, additional memory, called auxiliary memory or secondary storage, is used with most computer systems.
The secondary storage of a computer system is non-volatile.
It how low cost per bit stored, but it generally has an operating speed far slower that that of the primary storage.
This section of the memory is used to store large volume of data on a permanent basis, which can be partially transferred to the primary storage, as and when required for processing.

4. Commonly used secondary storage devices and their classification
Floppy Disk
Sequential Access Device
Direct Access Devices
Magnetic tape
WORM Disk
Optical Disks
Magnetic Disks
Zip Disk
Winchester Disk
Hard Disks
CD-ROM
Disk Pack
Secondary storage Devices

5. Sequential and Direct-Access Devices
Several different devices can be used as secondary storage device, but the one selected for a particular application, mainly depends upon how the stored information needs to be accessed.
There are two methods of accessing information
Sequential or several access, and
Direct or random access.

6. Magnetic Tape
Magnetic tape is the most popular storage medium for large data, which are sequentially accessed and processed.
The magnetic tape medium is a plastic ribbon, which is usually ½ inch or ¼ inch wide, and 50 to 2400 feet long.
It is coated with a magnetizable recording material, such as iron oxide or chromium dioxide.
Data are recorded on the tape in the form of tiny invisible magnetized and non-magnetized spots on the coated surface of the tape.

7. Magnetic Tape (cont…)
The tape ribbon is itself stored in reels or a small cartridge or cassette.
Example: like audio or video tape, the magnetic tape used in computer systems can also be erased and reused indefinitely.
Old data on a tape are automatically erased, as new data are recorded in the same area. However, the information stored can be read many times, without affecting the stored data.

8. Basic principles of operation
Storage Organization
Storage capacity
Data transfer Rate
Tape drive
Tape Controller

9. Storage capacity
The storage capacity of a tape is multiple of its length and data recorded density. That is,
Storage capacity of a tape = Data recording density X Length
Data recording density refers to the amount of data, which can be stored on a given length of tape.
It is measured in bytes per inch (bpi), or the number of bytes, which can be stored per linear inch of tape.
Tape density varies from 800 bpi in older systems to 77,000 bpi in some of the most modern systems.
Hence, if a 2400 feet tape has a data recording density of 800 bpi, its storage capacity will be
2400 x 12 inches x 800 bpi
= 23 x 106 bytes = 23 Mega bytes.
Note that, this is the total storage capacity of the tape.

10. Data Transfer Rate
Data Transfer Rate refers to the number of characters per second, which can be transmitted to the primary storage from the tape.
It is measured in bytes per second (bps).
Its value depends on the data recording density, and the speed with which the tape travels under the read/write head.
Tape speed is typically of the order of 100 inches per second.
Therefore, a tape having data recorded density of 77,000 bpi, and its drive having a tape speed of 100 inches per second, will have a data transfer rate of 77,000 x 100 = 77,00,000 bytes or 7.7 MB per second.

11. Tape Drive
A magnetic tape device is used for storage and retrieval of data, which is stored on magnetic tape medium.
The tape drive is different for tape reels, tape cartridges, and tape cassettes.
However, all of them work on a similar mechanism like the audio tape recorders or video cassettes recorders (VCR) found in our homes.
Instead of play and record, read and write commands are used with the tape drive.
Once loaded, the magnetic tape is said to be on-line; that is, it can now be used for storage or retrieval of data by the computer system.
When processing is complete, the tape is removed from the tape drive for off-line storage; that is, it is stored away from the computer system, and data on it are not accessible to the computer system, until it is loaded again on the tape drive.

12. Tape Controller
A magnetic tape must be mounted on a tape drive, before it can be used for reading/writing of information.
A tape drive is connected to, and controlled by a tape controller, which interprets the commands for operating the tape drive.

13. Type of Magnetic Tape
The data recording density and data transfer rate of magnetic tapes depend to a large extent on the data organization and principles of operation of various types of magnetic tapes, and their tapes drives. The commonly used ones are:  
½ -inch tape reel
½ - inch tape cartridge
¼ -inch streamer tape
4-mm digital audio tape (DAT)
They are described next slide

14. Advantages of Magnetic Tapes
Their storage capacity is virtually unlimited , because as many tapes, as required, can be used for storing very large data sets.
With the low cost of tape reels and cartridges, and high data recording densities, the cost per bit of storage is very low for magnetic tapes . An additional cost benefit is that tapes can be erased and reused many times.
Since the tape reels and cartridges are compact and light in weight, they are easy to handle and store. Very large amount of data can be stored in a small storage space.
Due to their compact size and lightweight tape reels and cartridges are also easily portable form one place to another . They are often used for transferring data and programs from one computer to another, which are not linked together.

15. Uses of Magnetic Tapes
Magnetic tapes are typically used for one or more of the following purposes:
For applications, which are based on sequential data processing. For example, as shown in Figure 8.7, the contents of an inventory master file (which is the permanent source of inventory data ) may be periodically updated using a transaction file ( which contains data reflecting inventory activities during the period ) to create a new inventory master file, which is used in the next processing cycle .
2.     Backing up of data stored on an on-line storage device, such as a disk, for its off-line storage, so that, if by accident, the data on the disk is corrupted or lost, it can be retrieved form the backup tape and stored back on the disk.
3.    Archiving of data which are not used frequently, but which may be used once in a while.
   

16. Limitations of Magnetic Tapes
1. Due to their sequential access nature, they are not suitable for storage of those data , which frequently require to be accessed randomly.
2. They must be stored in a dust-free environment, because specks of dust can cause tape reading errors.
3. They must also be stored in an environment with properly controlled temperature and humidity levels, otherwise, the tape ribbon may get twisted due to warping, resulting in loss of stored data.
4. They must be properly labeled, so that some useful data on a tape is not erased by mistake.

17. Magnetic Disks
Magnetic Disks are the most popular storage medium for direct-access secondary storage.
Due to their random access capability, magnetic Disks are the most popular on-line secondary storage device.
A magnetic Disks is a thin, circular plate/platter made or plastic, which is usually coated on both sides with a magnetizable recording material, such as iron oxide.
Data are recorded on the disk in the form of tiny invisible magnetized and non-magnetized spots on the coated surfaces of the disk.
A standard binary code, usually 8-bit EBCDIC, is used for recording data.
The disk itself is stored in a specially designed protective envelope or cartridge, or several of them may be stacked together in a sealed, contamination-free container.

18. Magnetic Disks
Like magnetic tapes, magnetic disks can also be erased and reused indefinitely.
Old data on a disk are automatically erased as new data are recorded in the same area.
However, the information stored can be read many times, without affecting the stored data.

19. Basic Principles of Operation
Storage Organization:
Track 000
200 tracks …
Track 199 …

20. Basic Principles of Operation
A sector

21. Storage Capacity
Storage capacity: The storage capacity of a disk is a multiple of the number of recording surfaces, number of tracks per surface, number of sectors per track, and number of bytes per sector. That is
Storage capacity of a disk system = Number of recording surfaces
X Number of tracks per surface X Number of sectors per track
X Number of bytes per sector
Example: let us assume that a disk pack has 10 disk plates, each having 2655 tracks. Also assume that there are 125 sectors per track, each sector can store 512 bytes.
Since the disk pack has 10 plates, it will have 18 recording surfaces.
Hence the capacity of the disk pack will be = 18 x 2655 x 125 x 512
= 3,05,85,60,000 bytes
= 3 x 109 bytes (approximately)

22. Cylinder
Lower
surface
not used

23. Type of magnetic Disks Floppy Disks Hard Disks.
All the magnetic disks are round platters.
They come in different sizes, different types kf packaging, and can be made of rigid metal or flexible plastic.
Based on these differences, there are many different types of magnetic disks available today.
However, all of them may be broadly classified into two types.
Floppy Disks
Hard Disks.
Winchester Disks
Magnetic Disks
Disk Packs
Floppy Disks
Hard Disks
Zip Disks

24. Floppy Disks
A floppy Disks is a round, flat plate of flexible plastic.
It is encased in a square plastic or vinyl jacket cover.
The jacket cover handling protection to the disk surface.
It has a special liner, which provides a wiping action to remove dust particles, which are harmful for the disk surface and the read/write head.

25. Floppy Disks Double density High density Very high density
Floppy disks are very cheap as compared to other secondary storage devices.
All 3 ½ floppy Disks are of double-sided type, and record data on both the disk surfaces.
They come in three different capacities
Double density
High density
Very high density

26. Storage Capacity of Floppy Disks
If the double-density 3 ½ inch diskettes have 40 tracks, 18 sectors/track, and 512bytes/sector. What is the total capacity of it?
Ans: 720 KB (approximately)

27. Storage Capacity of Floppy Disks
If the high-density 3 ½ inch diskettes have 80 tracks, 18 sectors/track, and 512bytes/sector. What is the total capacity of it?
Ans: 1.4 KB (approximately)

28. Storage Capacity of Floppy Disks
If the very high-density 3 ½ inch diskettes have 80 tracks, 36 sectors/track, and 512bytes/sector. What is the total capacity of it?
Ans: 2.88 MB (approximately)

29. Floppy Disks

30. Floppy-Disks Drive
A floppy-disk drive is a device, which is used to read/write data from/to floppy disks.
In case of currently used floppy disk drives, the rotational speed of a floppy-disk is of the order of 300 to 400rpm.
The data transfer rate is of the order of 10 to 30 Kilobytes/second.

31. Hard Disk
Hard disks are the primary on-line secondary storage device for most computer systems today.
Unlike floppy disks, which are made of flexible plastic or Mylar, hard disks are made of rigid metal.
The hard disk platters come in many sizes, ranging from 1 to 14-inch diameter.

32. Types of hard disks Zip/Bernoulli Disks Disk Packs Winchester Disks
Depending on how they are packaged, hard disks are normally categorized into following three types.
Zip/Bernoulli Disks
Disk Packs
Winchester Disks
Zip Disk
Winchester Disk
Hard Disks
Disk Pack

33.  The ZIP Disk
Moving up the popularity charts in secondary storage is the ZIP disk drive.
A single ZIP disk has the capacity of 70 floppy diskettes, that’s 100 MB for those of you in the back.
Larger files that are too big for a floppy are now easily stored and transported via the ZIP disk.
A ZIP drive may be of potable or fixed type.

34. Disk pack
A disk pack consists of multiple hard disk platters mounted on a single central shaft.
Hence, all the disks pack revolve together at the same speed.
As mentioned before, the disk drive of a disk pack has a separate read/write head foe each disk surface, excluding the upper surface of the top most disk, and the lower surface of the bottommost disk.
They are of removable/interchangeable type in the sense that they have to be mounted on the disk drive, before they can be used, and can be removed and kept-off-line, when not use.
That is, different disk packs can be mounted on the same disk-pack drive at different instances of time.
This gives virtually unlimited storage capacity to disk packs.

35. Lid of the plastic container
A disk pack
Lid of the plastic container
Plastic container
Hard disk platters

36. Winchester Disks
A Winchester disk also consists hard disk platters mounted on a single central shaft.
Howe ever, the main difference between a Winchester disk and a disk pack is that Winchester disks are fixed type. That is, hard disk platters and the disk drive are sealed together in a contamination free container and cannot be separated from each other.
Hence, as opposite to disk packs, which have virtually unlimited capacity, Winchester disks have limited capacity.

37. Winchester Disks
However for the same number of disk platters of the same size, Winchester disks can manage to have larger storage capacity than disk packs due to the following reasons:
Because both the disk platters and the disk drive are sealed in a contamination-free container and do not require to be separated later, all the surface of all the disk platters are used for data recording in case of Winchester disks.That is, for a Winchester disks with four platters, there are eight surfaces on which data can recorded, as opposed to six surfaces, in case of a disk pack with four platters.
The contamination-free environment allows Winchester disks to employ much greater precision of data recording and accessing, resulting in greater density of data storage than the interchangeable disk packs.
The storage capacity of today’s Winchester disks is usually of the order of a few tens of megabytes to a few gigabytes

38. Advantage and Limitations of Magnetic Disks
Advantages:
Unlike magnetic tapes, which support sequential access of data, magnetic disks support direct access of data. Hence they are more suitable for a wider range of applications.
Due to random access property, magnetic disks are often used simultaneously by multiple users as a shared device. For example, Winchester disks and disk packs are often used on-line secondary storage devices, in which case they store data of multiple users of the computer system. A tape is not suitable for such type of usage, due to its sequential-access property.
Magnetic disks are suitable for both on-line and off-line storage of data. For example, Winchester disks and disk packs are often used as on-line secondary storage devices, whereas floppy disks and zip disks are used as off-line secondary storage devices. In fact, the high-capacity Winchester disks have made it possible for today’s most personal computer users to enjoy the convenience of having all data and software readily accessible at all times.

39. Advantage of Magnetic Disks
Due to their low cost and high data recording densities, the cost per bit of storage is low for magnetic disks. An additional cost benefit is that magnetic disks can be erased and reused many times.
Floppy disks and zip disks are compact and high in weight. Hence, they are also easy to handle and store. Very large amount of data can be stored in a small space.
Due to compact size and light weight, floppy disks and zip disks are also easily portable from one place to another. They are often used for transferring data and programs from one computer to another, which are not linked together.

40. Advantage of Magnetic Disks (cont…)
Any information desired from a storage cane be accessed in a few milliseconds, because it its a direct access device. This is not possible in case of a tape storage, which is a sequential access storage device.
Data transfer rate a magnetic disk system is normally higher than a tape system.
Magnetic disks are less vulnerable to data corruption due to careless handling or unfavorable temperature and humidity conditions than magnetic tapes.
Except for the fixed type Winchester disks, the storage capacity of other magnetic disks is virtually unlimited, because as many disks as required can be used for storing very large data sets.

41. Limitations of Magnetic Disks
Although magnetic disks may be used for both types of applications (random as well as sequential data processing applications), for applications of t he latter type, use of magnetic disks may be less efficient than magnetic tapes.
It is more difficult to maintain the security of information stored on magnetic disks, which are used as shared, on-line secondary storage devices, as compared to information stored on magnetic tapes or on other types of magnetic disks.
For Winchester disks, a disk crash or drive often failure often results in the loss of entire data stored on it. It is not easy to recover the lost data. Hence, suitable backup procedures are suggested for data stored on a Winchester disks.

42. Limitations of Magnetic Disks
Some types of magnetic disks, such as disk packs and Winchester disks, are not so easily portable like magnetic tapes.
On a cost per bit basis, the cost of magnetic disks is low, but the cost of magnetic tapes is even lower
They must be stored in a dust-free environment.
Floppy disks, zip disks and disk packs should be labeled properly to prevent erasure of useful data by mistake.

43. Uses of Magnetic Disks
Magnetic disks are typically used one or more of the following purpose:
For applications that are based on random data processing.
As a shared, on-line secondary storage device. Winchester disks and packs are often used for this purpose.
A a backup device for off-line storage of data, so that if by accident, the data on an on-line storage device is corrupted, it can be retrieved from the backup storage. Floppy disks, zip disks, and disk packs are often used for this purpose.

44. Uses of Magnetic Disks
Archiving of data which are not used frequently, but which may be used once in a while. Floppy disks, zip disks, and disks packs are often used for this purpose.
Transferring of data and programs from one computer to another, which are not linked together. Floppy disks, zip disks are often used for this purpose.
Distribution of software by vendors. Originally sold software or software updates are often distributed by vendors on floppy disks and zip disks.

46. How much data can a HD DS disk and a Zip disk hold?
HD DS 3½” diskettes have capacity of 1.44 MB
Zip disks come in 100 MB, 250 MB, and 750 MB versions

47. How can ZIP disks store more data than standard floppy disk?
Disk density - closeness and size of magnetic particles on the disk’s surface.
Zip disks store data at a higher density than a standard 3½” floppy disk.

48. How does a hard disk work?
Hard disk platter - a flat, rigid disk made of aluminum or glass and coated with magnetic iron oxide particles
Density far exceeds floppy disk
Hard disk - one or more platters and their associated read-write heads
Preferred type of main storage
Miniature hard drives store 20 to 40 GB

49. How does a hard disk work?

50. What’s the downside of hard disk storage?
Head crash - when a read-write head runs into a dust particle or other contaminant on the disk
Head crash damages some data on disk
Triggered by jarring the hard disk while in use
Not limited to hard disks

51. Optical Disks
As compared to magnetic tape and magnetic disk, optical disk is a relatively new secondary storage medium.
During the last few years, it has proved to be a promising random disk access medium for high capacity secondary storage, because it can store extremely large amounts of data in a limited space.
An optical disk storage system consists of a rotating disk, which is coated with a thin metal or store other material that is highly reflective.
Laser beam technology is used for recording/reading of data on the disk. Due to the use of laser beam technology, optical disks are also known as laser disks or optical disks.

52. Basic principals of operation
Storage organization :
Unlike magnetic disks, which several concentric tracks, an optical disk has one long track, which starts at the outer edge and spirals inward to the center.
This spiral track is ideal for reading large block of sequential data, such as music. However, it makes for slower random access time the concentric tracks used by magnetic disks, whose sectors can be located faster, because they always found on a given track at a fixed distance from the center.
Magnetic disk
Optical disk

53. Storage capacity
The cost per bit storage is very low for optical disks, because of their low cost and enormous storage density.
They come in various sizes, ranging from 12-inch to 4.7-inch diameter. The most popular one is of 5.25 inch. Diameters, whose capacity is around 650 MB.
This storage capacity is equivalent to about 2,50,000 pages of printed text, or total capacity of 550 double sided, high-density floppy disk of the same size.

54. Storage capacity
As optical disks have a single track, their storage capacity is a multiple of the number of sector s, and number of bytes per sector, that is,
Storage capacity of an optical disk = Number of sectors x Number of bytes per sector
Example: The 5.25 inch optical disks typically have sectors, each of 2352 bytes.
 So the total capacity is x 2352 = 776 x 106 bytes = 776 MB.
 This is the unformatted capacity. The formatted capacity is about 650 MB

55. Optical disks drive:
An optical disk has a to be mounted on a n optical disk drive, before it can be use for reading or writing of information.
An optical disk drive contains all the mechanical electrical and electronic component for holding an optical disk, and for reading or writing of information on to it.
That is it contains the tray on which the disks is kept, the read/write beams assembly, and the motor to rotate the disks.
A typical optical disk drive is in figure books.

56. Type of the optical disks
All optical disks are round platters.
They come in difference sizes and capacities.
The two most popular types of optical disk in use today are
CD-ROM and
WORM disks.
  The are describe bellow:

57. CD-ROM CD ROM stands for compact Disk-Read Only Memory.
It is a spin-off of music. CD technology, and works much like the music CDs used in music systems.
In fact, if you have a soundboard and speakers connected to your computer, you can play music CDs your computer.  
The CD_ROM disk is a silver color metal disk of 5 ¼ inch diameter.

58. Storage Capacity
It has a storage capacity of about 650 MB. It is so called, because of its enormous storage capacity on a compact size disk, and because it is a read only storage medium.
That is these disks come pre-recorded, and the information stored on them cannot be altered.

59. WORM disk WORM stands for write-once, read-many.
WORM disks allow the users to create their own CD-ROM disks by using a CD recordable drive, which can be attached to a computer as a regular peripheral device.
WORM disks, which look like standard CD-ROM disks, are purchased blank and encoded using a CD-R drive.
The information recorded on a WORM disk by a CD-R drive can be read by any ordinary CD-ROM drive.

60. Advantages and Limitations of Optical Disks
The cost per bit of storage for Optical Disks is very low, because of their low cost and enormous storage capacity.
The use of a single spiral track makes Optical Disks an ideal storage medium for reading large blocks of sequential data, such as music.
Optical Disks drives do not have any mechanical read/write heads to rub against or crash into the disk surface. This makes optical disks a more reliable storage medium than magnetic tapes or magnetic disks.
Optical Disks have a data storage life in excess of 30 years. This makes them a better storage medium for data archiving as compared to magnetic tapes or magnetic disks.

61. Advantages and Limitations of Optical Disks
Since data once stored on an optical disk becomes permanent, the danger of stored data getting inadvertently erased/overwritten is not there with Optical Disks .
Due to their compact size and light weight, optical disks are easy to handle, store and port from one place to another.
Music CDs can be played on a computer having a CD-ROM drive along with a sound board and speakers. This allows computer systems to be also used as music system, whenever desired.

62. Limitations of Optical Disks
 It is a read-only storage medium. Data once recorded, cannot be erased, and hence, the optical disks cannot be reused. 
The data access speed for optical disks is slower than magnetic disks.
 Optical disks requires a more complicated drive mechanism than magnetic disks.

63. Use of Optical Disks
 The optical disks are typically used for one or more of the following purpose:
For distributing large amounts of data at low cost. For example, a complete encyclopedia, dictionary, world atlas, dictionary of quotations, biographies of great people , information about all educational institution of a particular type in a country, etc., are often distributed on CD-ROM disks.
For distribution of electronic of conference proceeding, journals, magazines, books, product catalogs, etc.
For distribution of new or upgraded versions of software products by software vendors

64. Use of Optical Disks
For storage and distribution of a wide Varity of multimedia applications, such as video games.
For achieving of data which are not used frequently, but which may be used once in a while.
End-user companies to make permanent storage of their own proprietary information often use WORM disks. For example, many banks use them for making A permanent record of their daily transactions.

65. Holds up to 80 minutes of music or 700 MB of data
CD and DVD technology: Is there a difference between CD and DVD technology? CD
Holds up to 80 minutes of music or 700 MB of data
DVD
Holds about 4.7 GB of data
A double layer DVD has two recordable layers on the same side and can store 8.5 GB of data

66. How do CD and DVD drives work?

67. How do CD and DVD drives work?
Recordable technology uses a laser to change the color in a dye layer sandwiched beneath the clear plastic disk surface
Rewritable technology uses “phase change” technology to alter a crystal structure on the disk surface

68. It is slower than hard disk access Not yet a suitable replacement
Are rewritable CD or DVD drives an acceptable replacement for a hard disk?
It is slower than hard disk access
Not yet a suitable replacement

69. Is my computer DVD drive the same as the one connected to my TV?
Not exactly
Movie files are still very large
Television DVD drives include MPEG decoding circuitry