Disk cache unit having reduced waiting times for data transfer

A disk cache control method performed by a disk cache controller for controlling transfer of data between an upper-rank processor, a disk unit and a cache memory which holds a copy of a portion of data stored on the disk unit, comprising the steps of:

• in response to an input/output command from the upper-rank processor requesting that desired data be read out from the disk unit, judging whether or not said desired data is stored in the cache memory;
• in response to a result of the judgement by the judging step, reading out data from the cache memory when said desired data is stored in the cache memory and controlling the disk unit so that a range of data that is requested by the input/output command is read out from the disk unit when said desired data is not stored in the cache memory;
• when a head of the disk unit is positioned to said desired data on the disk unit in accessing said desired data in the disk unit as requested by an input/output command from the upper-rank processor, determining by the disk cache controller whether or not a data transfer request can be conducted in a specified time period following completion of head positioning to transfer the data requested by the input/output command from said disk unit to said upper-rank processor, said specified time period being a time period necessary for reading out said desired data from the disk unit without waiting for a time period necessary for one rotation of a disk of the disk unit, said input/output command being a command for reading out from the disk unit said desired data which is required for data processing by the upper-rank processor;
• in response to a result of the determination by the determining step, transferring said desired data to the cache memory to be stored therein when the determining step determines that said desired data cannot be transferred, and thereafter, requesting the upper-rank processor to reissue said input/output command and reading out the data requested by the reissued input/output command from the cache memory to transfer said desired data to the upper-rank processor.

A disk cache control unit installed between an upper-rank processor and a disk unit comprising:

• a cache memory for holding a copy of a portion of data stored on the disk unit;
• judging means, responsive to an input/output command from the upper-rank processor requesting that desired data be read out from the disk unit, for judging whether or not said desired data is stored in the cache memory;
• means, responsive to a result of the judgment by the judging means, for reading out said desired data from the cache memory when said desired data is stored in the cache memory and transferring the read-out desired data from the cache memory to the upper-rank processor, and reading out said desired data from the disk unit when said desired data is not stored in the cache memory and transferring the read-out desired data to the upper-rank processor;
• means for determining whether or not a transfer of said desired data from the disk unit to the upper-rank processor can be completed during a time period necessary for reading out said desired data from the disk unit without waiting for a time period necessary for one rotation of a disk of the disk unit after a head of the disk unit is positioned to the desired data, said input/output command being a command for reading out from the disk unit said desired data which is required for data processing by the upper-rank processor;
• and
• means, responsive to a result of determination by the determining means, for transferring said desired data from the disk unit to the upper-rank processor if the means for determining determines that the transfer can be completed and storing said desired data in the cache memory and requesting the upper-rank processor to reissue said input/output command if the means for determining determines that the transfer cannot be completed.

A disk cache control method performed by a disk cache controller for controlling transfer of data between an upper-rank processor, a disk unit and a cache memory which holds a copy of a portion of data stored on the disk unit, comprising the steps of:

• in response to an input/output command from the upper-rank processor requesting that desired data be read out from the disk unit, judging whether or not said desired data is stored in the cache memory;
• in response to a result of the judgement by the judging step, reading out said desired data from the cache memory when said desired data is stored in the cache memory and transferring the read-out desired data from the cache memory to the upper-rank processor, and reading out said desired data from the disk unit when said desired data is not stored in the cache memory and transferring the read-out desired data to the upper-rank processor;
• when a head of the disk unit is positioned to said desired data on the disk unit in accessing said desired data in the disk unit as requested by an input/output command from the upper-rank processor, determining by the disk cache controller whether or not a data transfer of said desired data can be completed during a time period necessary for reading out said desired data from the disk unit without waiting for a time period necessary for one rotation of a disk of the disk unit after the head is positioned to said desired data, said input/output command being a command for reading out from the disk unit said desired data which is required for data processing by the upper-rank processor;
• in response to a result of the determination by the determining step, transferring said desired data from the disk unit to the upper-rank processor if the determining step determines that the transfer can be completed and storing said desired data in the cache memory and requesting the upper-rank processor to reissue said input/output command if the determining step determines that the transfer cannot be completed.