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Improved Write-ahead Logging

IP.com Disclosure Number: IPCOM000061164D
Original Publication Date: 1986-May-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Lindsay, BG: AUTHOR [+3]

Abstract

A method is described for improved write-ahead logging whereby efficient recovery can be achieved with a limited amount of increased logging during normal operation. The method comprises, in addition to write-ahead logging descriptions of data base modifications, the steps of (1) constantly monitoring the state of each buffer page with respect to its nonvolatile storage version; (2) periodically logging records of said states for all dirty pages; and (3) during restart, using the log of descriptions of data base modifications and the log of said record said states to determine whether modifications to a page need to be undone or redone.

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Improved Write-ahead Logging

A method is described for improved write-ahead logging whereby efficient recovery can be achieved with a limited amount of increased logging during normal operation. The method comprises, in addition to write-ahead logging descriptions of data base modifications, the steps of (1) constantly monitoring the state of each buffer page with respect to its nonvolatile storage version; (2) periodically logging records of said states for all dirty pages; and (3) during restart, using the log of descriptions of data base modifications and the log of said record said states to determine whether modifications to a page need to be undone or redone.

During data base recovery, it is necessary to determine the pages that were in buffers when the system crashed, and the earliest modify log records describing modifications to these pages which have not yet been written to nonvolatile storage. Efficient recovery can be achieved by limiting the number of pages which need to be examined for such determination during recovery. On the other hand, it is also desirable that this be done with a limited amount of increased logging during normal operation.

The method in this invention creates an individual Buffer Log Sequence Number (BLSN) for each buffer page. The BLSN of a page is initialized to the current LSN when the page is first fetched into the buffer. It is also updated to the current LSN whenever it is determined that all modifications to the page hav...