OS-Disk Performance Optimization

In multi programmed computing systems, inefficiency is often caused by improper use of rotational storage devices such as disks and drums.

This is a schematic representation of the side view of a moving-head disk. Data is recorded on a series of magnetic disk or platters. These disks are connected by a common spindle that spins at very high speed. The data is accessed (ie., either read or written) by a series of read-write heads, one head per disk surface. A read-write head can access only data immediately adjacent to it.

Therefore, before data can be accessed, the portion of the disk surface from which the data is to be read (or the portion on which the data is to be written) must rotate until it is immediately below (or above) the read-write head. The time it takes for data to rotate from its current position to a position adjacent to the read-write head is called latency time.

Each of the several read-write heads, while fixed in position, sketches out in circular track of data on a disk surface. All read-write heads are attached to a single boom or moving arm assembly. The boom may move in or out. When the boom moves the read-write heads to a new position, a different set of tracks becomes accessible. For a particular position of the boom, the set of tracks sketched out by all the read-write heads forms a vertical cylinder. The process of moving the boom to a new cylinder is called a seek operation.

Thus, in order to access a particular record of data on a moving-head disk, several operations are usually necessary. First, the boom must be moved to the appropriate cylinder. Then the portion of the disk on which the data record is stored must rotate until it is immediately under(or over) the read-write head (ie., latency time). 
Then the record, which is of arbitrary size must be made to spin by the read-write head. This is called transmission time. This is tediously slow compared with the high processing speeds of the central computer system.

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