2.5 Secondary storage and peripheral devices and architectures

< Free Open Study >

Memory storage volume is always looked at as an important feature when one thinks about acquiring a computer system. Whether the system is a desktop personal computer, a workstation, or a large special-purpose processor, data storage has always been a major selling point and a requested feature. As the price of memory has come down, the size of memory purchased for all classes of computers has gone up. One nonchanging feature is the general structure of the memory hierarchy. No matter how sophisticated or how simple the systems are, we will find that they all have something in common. The designers of the systems have organized data storage to maximize performance and provide adequate information volume storage.

The storage hierarchy (Figure 2.6) consists of a variety of data storage types that respond to the information needs of the system. From the highest-speed element (a cache) to the slowest-speed elements (archival devices), the tradeoff is the cost and speed of the storage medium per unit of memory. What is being attempted is to match the speed of the computer processor with the highest-speed devices within a reasonable cost curve. In the following sections we will examine the information storage devices outside of the central processing unit realm. This leaves out the high-speed expensive cache memories and primary memory. We will begin our review by looking at tape devices, magnetic disks, and archival devices.

click to expand
Figure 2.6: Memory hierarchy.

2.5.1 Tape storage devices

Magnetic tape information storage provides a low-cost, high-density storage medium for low-access or slow-access data. A tape unit consists of the storage medium (a spool of magnetic material formed into a tape), access electronics, and mechanical components (see Figure 2.7). A tape unit operates in a simple manner. Data on a tape can only be accessed in sequential form. Data must be located on the tape and then removed from the tape. A tape drive mechanically can rewind a tape, sequentially search the tape, and stop the tape. To access data stored on a tape an I/O program would have to command the tape unit to rewind the tape and then sequentially search the tape from the beginning until a match is found. Once found the addressed data can be removed.

Figure 2.7: Schematic diagram of a magnetic tape storage system.

To improve the performance of tape units, additional storage semantic access schemes have been devised. The beginning of the tape is reserved to maintain pointers to the start points of files stored on the tape. Instead of sequentially searching the entire tape, the controller searches the tape's directory, finds out where on the tape (e.g., how many feet from the directory region) the data are stored, and then uses this information to fast forward to the general location where linear search can resume. This allows for a speedup in the access and transfer of the data stored on the device-an important feature when a database management system is involved.

2.5.2 Magnetic and optical disk storage devices

An improvement over tape storage is the random access disk units, which most users of computers are aware of. The disks can be removable or internal fixed forms. A disk unit is typically comprised of one or more of the following: a controller, a movable access arm, and a magnetic storage medium in the form of a rotating platter (see Figure 2.8). The platter(s) is mounted on a spindle, which rotates at some given speed. The platter is organized into a set of rings called tracks and a partitioning of these tracks called sectors.

click to expand
Figure 2.8: Schematic diagram of a magnetic or optical disk system.

The movable arm contains the sensing and driving hardware to allow for the reading and writing of the magnetic or optical data stored on the platter. The controller orchestrates the access of the stored data based on a variety of access algorithms, only the simplest of which we will discuss here. The simplest form of disk access is that found in the sequential search paradigm. The disk controller knows on what sector and track a data file is stored and using this information the disk controller must perform some simple functions, such as moving the access arm out to the track the data are stored on (this is called seeking and the time it takes is called the seek time).

Once on the proper track, the controller must find the proper sector where the data are stored. This requires the controller to recognize the start of the sector markers on the track and to find the appropriate sector as it passes under the access arm's sensors. The time required for this is called the rotation time. Once the arm is over the proper sector and track, the data can be transferred from the medium to the controller. This time is called the transfer time.

So, for the average access of a data file on a disk we must take the following time:

(2.12)

One can readily see from this that the time to access data on a disk unit is greater than that of the primary memory and would typically be less than the time to extract a similar amount of data from a tape unit.

The density of the disk is based on the medium used to store the data. Disk units built on a magnetic medium are getting fairly dense, but they are approaching their limits. In addition, the medium is susceptible to failures due to airborne pollutants and magnetic fields. To improve this the industry has developed optical disk technology. This technology replaces the magnetic medium with an optical medium where data are stored as reflective optical media. The medium is similar to what is seen in television optical disk players.

2.5.3 Archival storage devices

Even with all of the disk and tape technology available, not all required data for a computer system can be kept on line. To keep data that are only occasionally needed we require archival storage devices. Archival storage devices typically have removable media. If you have access to the new multimedia systems or have a personal computer or workstation for use, you have interacted with a form of archival device: the removable disk, compact disk, or tape cartridge. This represents the most visible form of archival storage device. Data are loaded into the system as needed and removed when completed. The most recent archival storage device developed, the CD read/ write drive, has begun to blur the distinction between archival and on-line storage. Many systems use CD drives as enhanced storage for long-term applications memory. Some systems have even gone to the length where these represent the primary on-line storage.

Other, more elaborate, archival systems have been developed that use a combination of mechanical and electrical systems to port media on line and off line. These are similar to compact disk magazines and resemble jukeboxes. When a particular data item is needed, its physical storage location is found, and the medium is placed into the active storage hierarchy on line where the archived data can now be accessed. Again, this is a useful feature when we are talking about a very large database.

< Free Open Study >