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Incremental Data Base Log Image Copy

IP.com Disclosure Number: IPCOM000050906D
Original Publication Date: 1982-Dec-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Crus, RA: AUTHOR [+3]

Abstract

Traditional backup copies of data base data consist of all data in the object (data base or data set). The incremental image copy procedure herein described provides for copying only that data which has changed since the last image copy.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 53% of the total text.

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Incremental Data Base Log Image Copy

Traditional backup copies of data base data consist of all data in the object (data base or data set). The incremental image copy procedure herein described provides for copying only that data which has changed since the last image copy.

Although data base systems provide extensive controls on the consistency of the data, there are catastrophic situations from which recovery is not possible without a backup copy. For this reason, most (if not all) data base systems provide a means of taking a backup (or, image) copy of a data base or portion of a data base. As data bases tend to contain large amounts of data, the copying of the data takes large amounts of time. In general, to recover from a catastrophic failure, the recovery mechanism (1) restores the last backup copy and (2) reads the data base log and reapplies all necessary changes to bring the data back to its last consistent state.

If image copies are taken frequently, the time to perform the log process step is minimal. However, much valuable time is taken by the copy process. On the other hand, if copies are taken infrequently, the log process step can take enormous amounts of time.

One method to shorten the time of a backup copy is to partition the data base into smaller pieces from which copies are made. This allows each piece to be copied in a relatively short time, but all pieces must be copied frequently in order to minimize recovery time.

This article describes an image copy utility for a data base management system (DBMS) which allows the invoker to copy only selected pages (the unit of transfer between DASD and processor storage), while minimizing the effect on normal system operation.

Herein, the data base being managed by DBMS comprises objects. An object which contains data is called a 'tablespace'. A tablespace consists of pages. All pages of a tablespace are of the same size (such as either 4K or 32K bytes). The total size of a tablespace may be as large as 64GB (64 billion bytes). Most of the pages in a tablespace contain user data. However, in order to efficiently allocate space for new or updated records, some space map pages are reserved for system use.

A single space map page 'owns' many data pages, and contains two sections. The first section keeps track of how much space is available in each of the owned data pages. The second section contains a flag for e...