Re: log_block_checksum parameter

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: 2000/03/29
Message-ID: <954365791.25709.0.nnrp-12.9e984b29@news.demon.co.uk>#1/1

From the 8.1 reference manual init.ora parameters: db_block_checksum

If DB_BLOCK_CHECKSUM is set to TRUE, DBWn and the direct loader will calculate a checksum and store it in the cache header of every data block when
writing it to disk. Checksums will be verified when a block is read only if this
parameter is TRUE and the last write of the block stored a checksum.

If DB_BLOCK_CHECKSUM is set to TRUE, every log block will also be given a checksum before it is written to the current log.

The manuals have always said that checksumming is a significant overhead - but I've never been too convinced of the truth of that.

BTW - I believe that 8.1.6 does this for the SYSTEM tablespace at all times anyway, and as a consequence of a bug in 8.1.5 some databases when migrated from 8.1.5 to 8.1.6 suffer from a checksum failure on startup (and don't).

The benefit of the checksum in the log is that the archiver spots the problem, so you get a message about your archive stream being useless very promptly, and know that its time to take an emergency full backup - not sure quite how a db block checksum will help.

Perhaps T. Kyte can enlighten us ?

--

Jonathan Lewis
Yet another Oracle-related web site:  http://www.jlcomp.demon.co.uk

archie wrote in message <8btlla$iit$1_at_nnrp1.deja.com>...


>I've been going over some backup and recovery guides to help develop a

>backup and recovery strategy. I came across the log_block_checksum

>parameter to verify corrupt redo logs. Chapter 6 of the Oracle 8i

>Administrator's Guide (8.1.5 and 8.1.6) states that you should use this

>parameter to have Oracle compute a checksum for each redo log block. In

>the Appendix B of the 8i Migration Guide (8.1.5 and 8.1.6),

>log_block_checksum is listed as an obsolete parameter as of version 8.1.

>I was wondering if anyone knew why this parameter is obsolete and still

>included as a way to verify redo logs if you can't use it with version

>8.1.x?

>Does db_block_checksum, when set to true, perform this calculation

>instead?

>Thanks,

>John

>

>

>Sent via Deja.com http://www.deja.com/

>Before you buy.

Received on Wed Mar 29 2000 - 00:00:00 CST