Abstract: In a system having a host processor connected through a storage control unit to a cache store and a plurality of disk devices, segments of data which have been written to, while resident in the cache store, are transferred to the disks at some later time. If an abnormal condition, such as a bad spot on the disk, prevents the transfer of a segment from the cache store to the disk, an indicator is set for that segment by the storage control unit to prevent further attempts to transfer the segment. A segment whose indicator is set remains in the cache store until the host processor issues an initialize or a reset segment command.

Abstract: In a system having a cache memory and a bulk memory, and wherein a command queue is maintained for storing commands waiting to be executed, each command is assigned a priority level for execution vis-a-vis other commands in the queue. Commands are generated for transferring from the cache memory to the bulk memory segments of data which have been written to while resident in the cache memory. Each generated command may be assigned a generated priority level which is dependent upon the number of segments in the cache memory which have been written to but not yet copied into the bulk memory. A second priority level may be generated which is dependent on the time which has elapsed since the first write into any of the cache memory segments, and the priority level assigned to any given generated command is the higher of the two generated priority levels.

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.

Abstract: In a system wherein processors are connected through channel units to disk devices, two storage control units (SCU's) are provided between the channel units and the disk devices. The SCU's receive commands from the processor and issue seek instructions to the devices to locate the disk space specified by the commands, provided that a copy of the data from the specified disk space is not resident in a cache store. The commands may be queued for a time in a memory which is accessible by both SCU's. Before issuing a seek instruction to a device, an SCU first determines whether it is the only SCU having a path through a channel unit back to the processor which issued the command or whether there is a path from each SCU to the processor. If both SCU's have a path back to the processor, the SCU issues an untagged seek instruction to the device. If not, the SCU issues a tagged seek instruction to the device.