Disk Input and Output
Over the past 30 years, the increase in the speed of processors and main memory has far outstripped that of disk access, with processor and main memory speeds increasing by about two orders of magnitude compared with one order of magnitude for disk. The result is that the disks are currently at least four orders of magnitude slower than main memory.
The actual details of disk I/O operation depend on the computer system, the operating system, and the nature of the I/O channel and disk controller hardware. When the disk drive is operating, the disk is rotating at constant speed. To read or write, the head must be positioned at the desired track and at the beginning of the desired sector on that track. Track selection involves moving the head in a moveable head system or electronically selecting one head on a fixed head system. On a movable head system, the time it takes to position the head at the track is known as seek time. In either case, once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head. The time it takes for the beginning of the sector to reach the head is known as rotational delay, or rotational latency. The sum of the seek time if any and the rotational delay is the access time, the time it takes to get into position to read or write. Once the head is in position, the read or write operation is then operated as the sector moves under the heads this is the data transfer portion of the operation.
In addition to the access time and transfer time, several queuing delays are normally associated with a disk I/O operation. When a process issues an I/O request, it must first wait in a queue for the device to be available. At the time, the device is assigned to the process. If the device shares a single I/O channel or a set of I/O channels with other disk drives, then there may be an additional wait for the channel to be available. Seek is performed to begin disk access.
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In some large main frame systems, a technique known as RPS is used. When the seek command has been issued, the channel is released to handle other I/O operations. When seek is completed, the device determines when the data should rotate under the head.
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