Magnetic tape prefetch control system dynamically varying the size of the prefetched block

摘要

A magnetic tape system includes a driving unit (1) including a magnetic head (14), reels (11, 12) for winding a magnetic tape (16) thereon and a drive portion for driving the reels; a drive control unit (2, 20, 21) for controlling the drive unit; and a prefetch control unit (3) having a first memory for prefetching a plurality of commands from the host controller and a second memory for temporarily storing data from the host controller or data read out from the magnetic tape. The drive unit is operated through the drive control unit according to the commands stored in the first memory and the results of the operation are reported to the host controller. The prefetch control unit counts the number of blocks of write data of the write commands, and when the counted block number is within a free area of the second memory and is larger than a maximum block length (MALX) allowing prefetching in the second memory, the counted block number is set as the renewed maximum block length and prefetch control of the write data is executed based on the renewed maximum block length. When the drive control unit detects that the magnetic tape has arrived at the vicinity of the end thereof, the nunber (AN) of storable the write commands to be prefetched and stored in the first memory are reduced by the prefetch control unit.