Hierarchical memory system having cache/disk subsystem with command

Abstract

One or more host processors issue commands to one or more storage control units which control data transfers between the host processors, a cache store and a plurality of disk devices. A command queue is maintained for each disk device to store commands waiting to be executed by the disk device. The cache store stores segments of data which have been read from, or are to be written to disk space. In response to a command from a host processor a corresponding command is added to one of the command queues. If the disk device is not busy and has no previously queued commands waiting to be executed the storage control unit issues a seek command to the disk drive device. If there are previously queued commands waiting to be executed, or if the disk device is busy, the cache store is checked to determine if it contains a copy of the data from the disk space specified by the host processor command. If a copy of the data from the specified disk space is resident in the cache store then a data transfer is initiated between the host processor and the cache store. A priority value and a sequence number are assigned to each command as it is added to a queue so that the highest priority queued command with the lowest request number is executed when the disk device corresponding to the queue becomes idle. The storage control unit may create commands and place them on command queues for execution, these commands being for the purpose of transferring the least recently used segments of data from the cache store to the disks if the segments have been written to while in the cache store.