Sperry Corporation

Vladi Pusic

Robert E Swenson

Craig B Johnson

Lawrence D Sasscer

Benjamin T George

Monte E Smith

A data processing system has a host processor, a RAM, a cache memory for storing segments of data, a plurality of disk drive devices and a storage control unit for controlling data transfers, data from the host processor being written to the cache memory and subsequently destaged to the disks. The storage control unit continuously updates a variable indicating the number of written-to segments resident in the cache memory that have not been destaged to the disks. The variable is stored in the RAM. A File Status flipflop is responsive to the variable to produce a 'good file' signal when the variable is zero. The File Status flipflop is provided with an auxiliary power supply to maintain it in its present state in the event all power to the cache memory is terminated. Upon restoration of power after a complete power loss, the output of the status flipflop may be sampled by a command from the host processor to find out if any data was lost because written-to segments were present in the cache memory at the time all power was lost. The output of the File Status flipflop is also utilized to inhibit manually actuated functions which would destroy or invalidate written-to segments in the cache memory if the power to the cache memory were turned off, the cache memory were taken off line, or the port select switches between cache memory and the storage control unit were actuated to change port connections.

Cache/disk file status indicator with data protection feature

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.

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San Jose, CA, United States of America

Vladi Pusic