Digital Equipment Corporation

Compaq Computer Corporation, Inc.

William L Goleman

David William Thiel

Scott H Davis

Robert G Bean

James A Zahrobsky

Disclosed is a method and apparatus for accessing data in a computer system after a failed data operation in which I/O process state information is unknown. The failed data operation may cause data inconsistency among multiple devices associated with a shadow set for storing data. The disclosed system includes techniques for allowing continued data accesses while simultaneously re-establishing data consistency among members of the shadow set.

Method and apparatus for accessing data in a shadow set after a failed data operation

A log for managing data in a shadow set of storage media includes a system for maintaining a log of address information associated with at least one write command received from one of a plurality of data processing devices. The system for maintaining a log of address information includes a device for receiving a write command from a data processing device, a device which writes data associated with the write command in a section of one of the storage media, and a device which writes address information in a log indicative of the location of that section. A device is provided which then implements a management operation on data stored on one of the storage media in accordance with the address information stored in the log.

Log for selective management of specific address in a shadow storage

A method and apparatus for transferring data between two storage media, and is used in an illustrative embodiment to transfer data between two storage media in a shadow set, the storage media being accessible to one or more host processors. The method of the preferred embodiment includes the steps of: A. receiving a command from one of the host processors, the command specifying data to be transferred from a first storage medium to a second storage medium; B. transferring the data specified in the command received from the host from the first storage medium to the second storage medium in a series of subtransfers, each of the subtransfers transferring a portion of the data; and C. processing one or more I/O requests to the shadow set received from one or more host processors by, for each received I/O request: a. implementing the I/O request if the I/O request does not involve a section of the shadow set currently involved in one of the subtransfers; and b. stalling the I/O request if the I/O request involves a section of said shadow set currently involved in one of the subtransfers, and implementing the I/O request when the subtransfer has completed.

Transferring data in a digital data processing system

A digital data management system for managing a shadow set of storage media includes a plurality of storage media each accessible by at least one data processing device for I/O operations. Successive comparisons are carried out between data stored in corresponding locations in the storage media while maintaining access to the storage media for I/O operations. When inconsistency between data in corresponding locations is detected, a management operation is performed on at least one of the shadow set storage media. The management operation includes interrupting I/O operations to at least the storage medium on which the operation is performed, modifying data on one of the shadow set storage media to correct the inconsistency, and resuming availability of the storage media for I/O operations.

Digital data management system for maintaining consistency of data in a

A system and method for transferring data from a first storage medium to a second storage medium, each of the storage media being divided into corresponding data blocks, the method comprising steps of: (a) reading data stored in a first data block in the first storage medium, the first data block initially constituting a current data block; (b) comparing data read in the current data block to data stored in a corresponding data block in the second storage medium; (c) if the data compared in step b are identical, reading data stored in a different data block in the first storage medium, the different data block becoming the current data block, and returning to step b; (d) modifying the data stored in one of the storage media such that the data in the current data block is identical to the corresponding data in the second storage medium; and (e) rereading the data in the current data block and returning to step b.

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Nashua, NH, United States of America

William L Goleman