Dynamic information storage or retrieval – Information location or remote operator actuated control – Selective addressing of storage medium
Patent
1992-04-29
1993-12-07
Look, Edward K.
Dynamic information storage or retrieval
Information location or remote operator actuated control
Selective addressing of storage medium
369215, 360 7804, 395275, G11B 2108
Patent
active
052688844
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a seek control method and apparatus for an auxiliary storage device in a computer system.
Recently, it has been required that an auxiliary storage device which stores a large amount of data and which is used as a virtual memory operates at a higher speed as a computer main frame operates at a higher speed. In order to meet this requirement, it is necessary to reduce an access time of the auxiliary storage device. In order to reduce the access time, it is necessary to carry out a seek control directed to reducing a seek time.
Background Art
A conventional magnetic disk device or a conventional optical disk device, which serves as an auxiliary storage device in a computer system, has a possibility that even when a command to read data is issued by a high-order device during a seek operation in which a head is moving from a cylinder (track) to another track, this command is not allowed to be carried out until the seek operation is completed.
The above will be described in the case of a magnetic disk apparatus. Normally, the magnetic disk apparatus employs a method called a read ahead cache. For example, readout data is transferred to a high-order device such as a host computer, in response to a data readout command from the high-order device, and thereafter data subsequent to the above readout data is read ahead and written into a cache memory provided in a controller of the magnetic disk apparatus (read ahead cache mechanism). If there is a continuous defect in an area of a magnetic disk medium read out by the read command during such a read ahead operation, an alternative process is carried out in which an alternative area of the defective area is provided in another cylinder. When consecutive data are read ahead, a seek operation for moving the head to the alternative area is carried out. The alternative process as described above is disclosed in U.S. Pat. 4,498,146. However, in many cases, the alternative cylinder is assigned on the outermost or innermost periphery side of the magnetic disk other than cylinders which are used most frequently. Thus, the seeking from the read ahead area having a defect to the alternative area uses a long distance.
When such a long-distance seek is carried out, the magnetic head is moved at a high speed in order to reduce the seek time. Thus, when the moving speed of the magnetic head becomes equal to or higher than a certain speed, a magnetic disk controller cannot accept a command from the high-order device due to the control of the movement of the magnetic disk for the following reason.
As indicated by F shown in FIG. 1, long-distance seeking is controlled so that the moving speed of the magnetic head becomes equal to a target speed. That is, the difference between a cylinder on which the magnetic head is now positioned and a target cylinder is obtained, and a target speed curve making it possible for the magnetic head to need the shortest time to move in accordance with the obtained difference is determined. Then, the movement control to the head is carried out while a speed control is being performed, so that the real speed of the magnetic head is the same as the speed defined by the target speed curve. This speed control is carried out by a microprocessor (MPU) in the magnetic disk controller. The MPU always detects the current position of the magnetic head and outputs, to a servo control circuit, the number of remaining cylinders necessary to move the magnetic head from the current position to the target position by detecting a track crossing pulses (generated each time the head passes over each track). Assuming now that each track (cylinder) is 17.5 .mu.m wide and the maximum speed of the magnetic head is 2 m/s, it takes 8.75 .mu.s for the magnetic head to cross each track. Thus, in order to read the track crossing pulse, it is necessary to monitor the track crossing pulse at intervals shorter than the above-mentioned track crossing time.
As has been described above, it is necessary t
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patent: 4691310 (1987-09-01), Wine
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patent: 4925828 (1990-06-01), Frissell
patent: 5206943 (1993-04-01), Callison et al.
Hatagami Toshifumi
Kiuchi Tsutomu
Moriyoshi Shuhei
Fujitsu Limited
Lee Michael S.
Look Edward K.
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