Comments on: Excess redo log generation during Hot Backup – Oracle 9i

By: sanjeev

sanjeev — Thu, 21 Jul 2011 11:18:24 +0000

That is great Advait, its really wonderful description on accessive redo generation.

By: asha singh

asha singh — Thu, 21 Oct 2010 11:16:05 +0000

Differnce between full backup, whole backup & partial Backup

By: Sreenivas

Sreenivas — Sat, 07 Aug 2010 10:01:43 +0000

Nice explanation.thanks

By: ramesh

ramesh — Tue, 20 Jul 2010 18:17:31 +0000

I have one doubt regarding when tablespace or datafile is in offline
what is the bagground process how to see

By: Sagar B V

Sagar B V — Mon, 01 Feb 2010 15:40:47 +0000

Hi everyone,

Really good explanation for junior DBA’s…!

Thanks a lot

By: akhil mahajan

akhil mahajan — Mon, 27 Jul 2009 07:17:27 +0000

Originally posted by the article writer
“Question arises, why Oracle has to copy the complete block to redo log files. As you have seen above that during copy of datafile, there can be many fractured blocks, and during restore and recovery its going to put those block back and try to recover. Now assume that block is fractured and oracle has minimum information that it generates in the redo. Under such condition it wont be possible for Oracle to recover such blocks. So instead Oracle just copies the entire block back from redo log files to datafiles during recovery process. This will make the datafile consistent. So recovery process is very important which takes care of all fractured blocks and makes it possible to recover a database.”

The question is still unanswered. Why can’t the Oracle just go on to apply the changed entries in the block and why is the redo of whole block generated? I understand that the redo generated is minimal but we actually don’t need the redo of whole block. Isn’t it?
I have never come across the right explanation for this question. I hope u ll help me.

By: jeevan

jeevan — Sat, 30 May 2009 18:00:36 +0000

Hi all masters,

I have a doubt,since data gets written in to the datafiles,why should the complete blocks get written in to the redologs.Is there any use with that stuff?
Why can’t the redolog process go as asusual ?

Regards
Jeevan

By: Nirnay Jaiswal

Nirnay Jaiswal — Mon, 23 Mar 2009 08:38:36 +0000

Hi Prthvira,

Nothing is written to datafiles from archived redo logs, after the hot backup ends. Only the datafile header is updated with the current scn.

Now, why it is compulsory to put database in archive log mode during hot backup? Because when you start copying the datafile thru O/S commands it takes some time for copying the file, say at time t1 you start to copy the file datafile ‘dat01.dbf’ to ‘dat01_bak.dbf’ and the SCN was SCN1 and when the copying was finished at time t2 then the SCN has changed to SCN1 + N. So you have blocks in your copied file ‘dat01_bak.dbf’ which have the SCN ranging from SCN1 to SCN1 + N. But all the blocks have to be with the same SCN to be consistent, and to be used. So to make the backup copy file ‘dat01_bak.dbf’ consistent, you need to have the archive logs starting from SCN1 to SCN1 + N.

—— Hope this Helps ——-

Nirnay Jaiswal

By: sridhar

sridhar — Tue, 03 Feb 2009 14:56:05 +0000

HI,

i want to know y do we apply arichive before media recovery.

Thanks,
kola.sreedhar

By: prthvira

prthvira — Thu, 30 Oct 2008 13:32:24 +0000

great information, but one query, once we end hot backups,SCN in datafile header is updated to current scn, at that time do any changes, inserts happened during backup are written to datafile from archived redo logs. if not then why it is compulsary to put database in archive log mode during hot backups?.(i am not concerned about restore and recover but only about hot backup process)