Difference between Physical and Logical Logs?

Posted in 2007

Summary

A newcomer asked how Informix's physical log differs from the logical log, and why a physical log is needed if the logical log already holds pre- and post-images. Answers: the physical log stores pre-images of pages modified since the last checkpoint, is emptied at each checkpoint (75% full forces an early checkpoint), is copied into archives, and on crash recovery is written back to disk to give fast recovery/logical log rollforward a consistent starting point. The logical log records data, schema and meta-data changes for rollback, restore rollforward and recovery. Readers were also pointed to the Administrator's Guide (chapters 14-17).

Auto-generated by DrWatson from the posts below — may be imperfect; read the full thread.

Topics: Logging & Checkpoints

DANNY NG — Fri, 28 Dec 2007 08:22:39 — source: IIUG Forums & Mailing Lists

I am a new user of Informix. I would like to know the difference between physical and logical logs in informix dataserver?

↪ replying to DANNY NG

Jack Parker — Fri, 28 Dec 2007 13:15:21 — source: IIUG Forums & Mailing Lists

I'd recommend reading the administrator guide which discusses this far more clearly than anyone here can do. You can find it at: http://publib.boulder.ibm.com/epubs/pdf/25122672.pdf. (Note this is v10 doc. - you can navigate to the desired documentation starting at: http://www-306.ibm.com/software/data/informix/pubs/library/). Chapters 14-17. j. -----Original Message----- From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org]On Behalf Of DANNY NG Sent: Friday, December 28, 2007 3:23 AM To: ids@iiug.org Subject: Difference between Physical and Logical Logs? [10808] I am a new user of Informix. I would like to know the difference between physical and logical logs in informix dataserver? **************************************************************************** *** Forum Note: Use "Reply" to post a response in the discussion forum. See you at the IIUG Informix 2008 Conference The Power Conference for Informix Professionals April 27 - 30, 2008 Marriott Overland Park (Kansas City), Kansas http://www.iiug.org/conf Registration Now Open!!

↪ replying to DANNY NG

Art S. Kagel — Fri, 28 Dec 2007 14:32:44 — source: IIUG Forums & Mailing Lists

DANNY NG wrote: > I am a new user of Informix. I would like to know the difference between > physical and logical logs in informix dataserver? > Danny, take the suggestion and read the Administrators Guide, but here's the quick run down: Physical logs: Keeps pre-images of of database pages that have been modified since the last checkpoint. Emptied at the end of each checkpoint. Filling the physical log past 75% full will force an early checkpoint to preserve server integrity. Logical logs: Records modifications to server objects including meta-data, schema, and data records. Includes both pre- and post- images as appropriate to the change. Used for transaction rollback, archive rollforward during a restore, rollforward and rollback of incomplete transactions during crash recovery (and in IDS 11.10+ LAST COMMITTED isolation query processing). Art S. Kagel > > ******************************************************************************* > Forum Note: Use "Reply" to post a response in the discussion forum. > > See you at the IIUG Informix 2008 Conference > The Power Conference for Informix Professionals > April 27 - 30, 2008 Marriott Overland Park (Kansas City), Kansas > http://www.iiug.org/conf > Registration Now Open!! > >

↪ replying to Art S. Kagel

DANNY NG — Mon, 31 Dec 2007 00:58:12 — source: IIUG Forums & Mailing Lists

It seems the work done by logical log is more than physical log. What is the point of having physical log? As logical log already including post and pre image of database chanages.

↪ replying to DANNY NG

Art S. Kagel — Mon, 31 Dec 2007 02:21:56 — source: IIUG Forums & Mailing Lists

DANNY NG wrote: > It seems the work done by logical log is more than physical log. What is the > point of having physical log? As logical log already including post and pre > image of database chanages. > The physical log is ONLY used in two situations: 1- The contents are copied to server archives so that the archive will contain the versions of pages modified during the archive process that are as of the beginning of the archive. 2- After a server or system crash the contents of the physical log are written back to disk so that the logical log rollforward/rollback that is the larger part of the fast recovery procedure have a clean slate to write to. This is in case any partial pages were flushed during the crash since the server page size (2-32K) is larger than the physical storage unit of the drives (512 bytes). Again, read the Administrator's Guide where all of this is explained in detail. Art S. Kagel

↪ replying to DANNY NG

Madison Pruet — Mon, 31 Dec 2007 03:23:45 — source: IIUG Forums & Mailing Lists

Basically to give the logical log a starting point to apply the logs. ------------------------------------- Madison Pruet, STSM IDS Replication Architect = "DANNY NG" = <nd504@ha.org.hk> = Sent by: = To ids-bounces@iiug. ids@iiug.org = org = cc = Subj= ect 12/30/2007 06:58 Re: Difference between Physical = and PM Logical Logs? [10813] = = = Please respond to = ids@iiug.org = = = It seems the work done by logical log is more than physical log. What i= s the point of having physical log? As logical log already including post and= pre image of database chanages. ***********************************************************************= ******** Forum Note: Use "Reply" to post a response in the discussion forum. See you at the IIUG Informix 2008 Conference The Power Conference for Informix Professionals April 27 - 30, 2008 Marriott Overland Park (Kansas City), Kansas http://www.iiug.org/conf Registration Now Open!! =

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