Although there are several different devices that one can use as a secondary storage devices, the one selected for a particular application depends mainly upon how the application needs to access the stored information. These are two methods of accessing information- sequential or serial access, and direct or random access.
A sequential-access storage device is one in which arrival at a desired storage location is preceded by sequencing through other locations so that access time various 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 case of a sequential-access device, if we need to access an address an address that is out of order, we can reach at it only by searching through all the address 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. This is analogous to a music tape cassette. If 10 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 quickly past the first seven songs, but the first seven songs are still accessed, although, not fully 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 one after another. Preparations of monthly pay slips, monthly electricity bills, etc., are examples of such applications. These applications process data record data records of every employee or customer at scheduled intervals (in this case monthly).
Magnetic tape is an example of a sequential-access storage device.
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. This is analogous to a music CD. iF 10 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 eighth 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 the song.
Direct-access devices are useful for such applications, which need to access information in a more direct manner 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 saving account. Similarly, in a computerized airline ticket booking system, we may require immediate access to reservation system records to find out 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 long enough time to cause frustration to the customer.
Magnetic disks, optical disks, and memory storage devices are examples of direct-access storage devices.