Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Patent
1997-01-22
2000-05-09
Nguyen, Hiep T
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
707202, 707204, 714 6, 714 20, 713502, G06F 1216
Patent
active
060617691
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to distributed data processing and more specifically to the taking of backup copies of data sets that are open for update by multiple updaters.
BACKGROUND OF THE INVENTION
In data processing systems, the failures that can occur include communication failures (in online systems), data set or database failures, application or system program failures, processor failures and power supply failures. All these problems are potentially more severe in an online system than in a system that performs only batch processing.
In batch systems, input data is usually prepared before processing begins, and jobs can be rerun, either from the start of the job or from some intermediate checkpoint. In online systems, input is usually created dynamically by terminal operators, and arrives in an unpredictable sequence from many different sources. If a failure occurs, it is generally not possible simply to rerun the application because the content and sequence of the input data is unknown. And, even if it is known, it is usually impractical for operators to reenter a day's work.
Online applications therefore require a system with special mechanisms for recovery and restart that batch systems do not require. These mechanisms ensure that each data set (resource) associated with an interrupted online application returns to a known state so that processing can restart safely.
An online system requires mechanisms that, together with suitable operating procedures, provide automatic recovery from failures and allow the system to restart with the minimum of disruption.
The two main recovery requirements of an online system are to maintain the integrity of data and to minimize the effect of failures.
Maintaining the integrity of the data means that the data is in the form you expect and has not been corrupted. The object of recovery operations on data sets, databases, and similar data resources is to maintain, and restore, the integrity of the information. Ideally, it should be possible to restore the data to a consistent, known, state following any type of failure, with a minimum loss of previous valid updating activity.
One way of doing this is to keep a record, or log, of all the changes made to a resource while the system is executing normally. If a failure occurs, the logged information can help recover the data.
The information can be used in two ways:
1. It can be used to back out incomplete or invalid changes to one or more resources. This is called backward recovery, or backout. For backout, it is necessary to record the contents of a data element before it is changed. These records are called before images. In general, backout is applicable to processing failures that prevent one or more transactions (or a batch program) from completing.
2. It can be used to reconstruct changes to a resource, starting with a backup copy of the resource taken earlier. This is called forward recovery. For forward recovery, it is necessary to record the contents of a data element after it is changed. These records are called after images. In general, forward recovery is applicable to data set failures, or failures in similar data resources, that cause data to become unusable because it has been corrupted or because the physical storage medium has been damaged.
In many cases, a data set failure also causes a processing failure. Then, forward recovery must be followed by backward recovery.
In some environments, a data set might need to remain online and open for update for extended periods. Normally, a backup copy of the data set cannot be taken while the data set is open. Thus, if a failure occurs that requires forward recovery, all updates that have been made to the data set since it was opened must be recovered. This means that all forward recovery logs that have been produced since the data set was opened must be kept. For a heavily-used data set that has been open for update for several days or weeks, much forward recovery could be needed.
Because of the above considerations, it was desira
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Kapulka Kenneth Michael
Rose Ian Hulme
Scott Jonathan Andrew
Strickland Jimmy Paul
International Business Machines - Corporation
Klein Esther E.
Krall Noreen A.
Nguyen Hiep T
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