RE: Online Redo Log During a Hot Backup

What in the world are you talking about?

Presumably by hot backup you mean a physical backup not through RMAN that is initiated performed on a tablespace by altering a tablespace to backup mode.

Excuse the rest if that is not what you are talking about.

When this command is executed, then the next block change for each block that is changed in the files making up the tablespace is logged as a whole block, so that if the Oracle block is fractured by the OS utility executing the copy to backup, then recovery is guaranteed to have a full block image from which to roll forward. Since database writers are not required to be synchronous with transactions, but rather log writer is the guardian of committed information, there is no guarantee that a block is up to date and not fractured after restoration from backup other than reaching the point in the log that matches the recording of the tablespace (and each file composing the tablespace) as no longer in backup. None of this prevents filled online redo logs from being processed by the archiver. They just have the possibility of being more data than they would have been for the same transactions, since some blocks may be logged where otherwise only the change to the block would have been required.

For purposes of recovery, this just means that if you want to execute a point in time recovery to a point between a begin backup and an end backup, then you must use an older vintage of backup of the file concerned.

For purposes of having a backup set that can be completely recovered, it means that after the end backup command for the last tablespace of a backup set is complete, you need to switch log files, archive that log containing the end backup, and make sure that you have the archived logs containing the start of backup of the first tablespace through the end backup of the last tablespace taken out of backup.

Restoration operates perfectly heterochronously as regards the starting vintage of the files composing tablespaces, so if you want to do a point in time
recovery to a point when only some of the tablespaces are in backup, then you only need the older vintage starting point for those tablespaces. (Of course you'll also need the archived logs to bring those files up to the point you need.

Now if you doubt me there is a simple low cost test.

select * from v$log;
alter some_tablespace_name begin backup; alter system switch logfile;
<wait a bit>
select * from v$log;

and you will see that the log is archived. Cycle through all the logs if you must and you'll see it does not hang and unless there is some problem with your archiver or destination, the archived redo logs will appear where they are supposed to appear like clockwork. I'm not sure whether Rama wrote something that confused you or if you got this information from somewhere else.

Being in backup mode for one or more tablespaces simply does not interfere with archiving log files in any way that I can determine. Hot backup would be pretty useless if it did and it would then require that you had enough online logs to handle the entire change during the period of backup.

mwf

-----Original Message-----
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of aamgm service Sent: Thursday, September 23, 2004 1:19 AM To: oracle-l_at_freelists.org
Subject: RE: Online Redo Log During a Hot Backup

the 2-3 hours is a tape backup of a 80GB database. My confusion was that, Oracle will ONLY archive checkpointed online redo logs and since "checkpointing" online redo logs is suspended when a database is in backup mode then filled online redos are not archived! 1. So how the does oracle apply changes made during the hot backup when the redo logs have been archived?
2. What is the recommended procedure of taking the database in backup up mode, I mean put all tablespaces in backup mode at once or one tablespace at a time?

Thank you Gurus for all the explanations

Regards

> -----Original Message-----
> From: Hostetter, Jay M [SMTP:JHostetter_at_decommunications.com]
> Sent: Wednesday, September 22, 2004 2:25 PM
> To: oracle-l_at_freelists.org
> Subject: RE: Online Redo Log During a Hot Backup
>
> Page 68 and 69 of Velpuri's "Backup and Recovery Handbook" gives a good
> explanation of what goes on internally when tablespaces are in hot
> backup mode. If you are in archivelog mode and you are automatically
> archiving your log files, you don't have anything to worry about. Hot
> backup mode just increases redo activity. Redo logs are still archived
> during a hot backup.
> Is the "2-3 hours" for a tape backup of your system? Or is it a script
> that backs up your database by putting each tablespace in backup mode?
>
> Jay=20
>
> -----Original Message-----
> From: oracle-l-bounce_at_freelists.org
> [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of aamgm service
> Sent: Wednesday, September 22, 2004 6:42 AM
> To: 'oracle-l_at_freelists.org'
> Subject: Online Redo Log During a Hot Backup
>
> Dear Gurus,
>
> I have been running my database in noarchive mode and I want to switch
> to Archivelog mode. I am running Oracle 7.3.4. I have three groups of
> redo logs that are 20MB each.=20
> My question is what happens to filled online redo logs during a
> hot/online backup?? My fear is my database could hang when all redo logs
> fill up before my backup completes because it usually takes 2-3 hours
> (backup) to complete?
> and my log switches occur in 15-20 mins.
>
> TIA
> Regards
> =20
>
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