Hard Disk Performance
The storage medium in a magnetic-disk system consists of one or more disks mounted on a common spindle. A thin magnetic film is deposited on each disk, usually on both sides. The disks are placed in a rotary drive so that the magnetized surfaces move in close proximity to read/write heads. The disk rotates at a uniform speed. The following factors determine the performance of a hard disk.
Access Time: Disk access time is the interval between the time a computer makes a request for transfer of data from a disk system to the primary storage and the time this operation is completed. To access information stored on a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of the surface number track/cylinder number and sector number. Information is always written from the beginning of a sector and can be read only from the track beginning. Hence disk access time depends on the following three parameters.
1. Seek time - As soon as a read/write command is received by the disk unit, the read/write heads are first positioned on to the specific track number by moving the access arms assembly in the proper direction. The time required to position the read/write head over the desired track is called the seek time. The seek time varies depending on the position of the access arms assembly when a read/write command is received. If the access arms assembly is positioned on the outer most track and track to be reached is the inner most one the seek time will be maximum. It will be zero if the access arms assembly already happens to be on the desired track. The average seek time is thus specified for most systems. It is of the order of 10 to 100 milliseconds.
We also know that some disk systems have multiple read/write heads on each access arm. This is done to reduce the seek time. For example: a disk system may have two sets of read/write heads for each surface one for reading/writing on the inside tracks and another for the outside tracks. This will reduce the average seek time by half because each read/write head needs to cover and move across only half of the total number of tracks.
2. Latency- Once the heads are positioned on the desired track the head on the specified surface is activated. Since the disk is continuously rotating this head should wait for the desired data to come under it. This rotational waiting time i.e., the time required to spin the desired sector under the head is called the latency. The latency also known as the rotational delay time is also a variable and depends on the distance of the desired sector from the initial position of the head on the specified track. It also depends on the rotational speed of the disk which may be anywhere from 300 rpm to 7200 rpm. An average latency time is thus normally specified which is of the order of 5 to 80 milliseconds. Note that the average latency of a disk system is equal to half the time taken by the disk to rotate once. Hence the average latency of a disk system whose rotational speed is 3600 rpm will be 0.5/3600 minute=8.3 milliseconds.
3. Transfer rate -Transfer rate refers to the rate at which data are read from or written to the disk. Once the read/write head is positioned over the desired sector the data are read, written at a speed determined by the rotational speed of the disk. If the rotational speed of a disk is 3600 rpm and the disk has 125 sectors/track and 512 bytes/sector the amount of data transferred in one full revolution of the disk will be 125*512=64000 bytes=64k bytes(approximately). Hence the transfer rate of the disk system will be 64,000 * 3600/60 bytes/second = 38,40,000 bytes/second = 3.8 megabytes/second. Notice that the transfer rate of a disk system depends on the density of the stored data and the rotational speed of the disk.
Since the data transfer time is negligible as compared to seek time and latency the average access time for a disk system is the sum of its average seek time and average latency. The average access time for different types of disk systems varies over a wide range and may be anywhere from 10 to 600 milliseconds.
Since the access time for a piece of data stored on a disk depends on the physical location of the data it is more correct to refer to a disk system as direct access storage device instead of random access storage device. Random access refers to a storage device in which the access the access time is independent of the physical location of the data. For example primary storage is a random access storage. However this distinction is not always observed strictly and hence disk systems are often referred to as random access storage devices.
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