Secondary Storage Devices
The primary storage of today’s computer systems has the following limitations:
1. Limited capacity: It is often necessary to store many millions, sometimes billions, and even trillions, of bytes of data in a computer. Unfortunately, the primary storage capacity of today’s computers is insufficient to store the large volume of data handled by most data processing centres.
2. Volatile: Primary storage is volatile and loses data stored in it in case of power off or interruption of power. However, computer systems must keep data permanently for several days, months, or even years.
Almost all computers use additional memory called Auxiliary memory or Secondary storage to overcome these limitations of primary storage.
Secondary storage is non-volatile and has a lower cost per bit but generally operates at speeds far slower than that of primary storage. Computer systems use it primarily to permanently store a large volume of data, which they transfer partially to primary storage whenever processing requires.
Over the years, computer systems have used several devices and media as secondary storage. Many of them have become outdated, such as punched paper tape, punch cards, and floppy disks.
Sequential-Access And Direct-Access Devices
Although several different devices can be used as a secondary storage devices, the one selected for a particular application depends mainly on how the application needs to access the stored information.
There are two methods of accessing information: -
1. Sequential or Serial Access
2. Direct or Random Access
Sequential-Access Devices
A sequential-access storage device is one where arrival at the desired storage location is preceded by sequencing through another location so that access time varies according to location. That is, we can retrieve information stored on a sequential-access device only in the same sequence in which it is stored.
In the case of a sequential-access device, if we need to access an address that is out of order, we can reach it only by searching through all the addresses stored before it. For instance, we cannot access data stored at the last few locations until we have traversed all preceding locations in the sequence.
It is analogous to a music tape cassette. If ten songs are recorded on a music tape, and if we want to listen to (or access) the eighth song, we must first listen to (or access) the seven songs coming before it. The player may “fast forward” the tape past the first seven songs, but the first seven songs are still accessed, although not entirely played.
In the same way, to access the 28th data record stored on a sequential-access device, a computer must first access (although not process) the first 27 data records. Sequential access devices are suitable for storing data of such applications, which need to process most (if not all) data records.
Examples of such applications are preparations of monthly pay slips, monthly electricity bills, etc. These applications process data records of every employee or customer at scheduled intervals (in this case, monthly).
An example of a sequential-access storage device is Magnetic tape.
Direct-Access Devices
On the other hand, a Direct-access or Random-access storage device is one in which we can reach and access any storage location at random, and approximately equal access time is required for accessing each location.
It is analogous to a music CD. If ten songs are recorded on a music CD, and if we want to listen to (or access) the eighth song, we need not listen to or fast-forward the seven songs coming before it. All we have to do is select track number eight on-the player, and the player moves the pickup arm directly across the CD to the groove where the eighth song begins and starts playing.
Direct-access devices are helpful for such applications, which need to access information more instantly than sequential-access devices allow.
For example, in a computerized bank, at any instance, we may require to determine the exact balance in a customer’s savings account. Similarly, in an airline ticket booking system, we may need immediate access to reservation system records to determine if seats are available on a particular flight.
In such applications, if we use a sequential-access device for data storage, accessing desired information may take a long enough time to cause frustration to the customer.
Magnetic disks, Optical disks, and Memory storage devices are examples of Direct-access storage devices.
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