Re: Backup Oracle ? Online Redo Log.

In article <3c3da31b$0$24015$4d4eb98e_at_read.news.fr.uu.net>, "Christian says...
>
>Thanks for your answer !
>
>I've read the file (.PDF) which explain backup and recovery but I not sure
>understanding why the online redolog files should never be backuped in a hot
>backup.
>
>For example :
>If I have a HOT BACKUP of :
> - the whole database at 03h00 am
> - the control files,archived logs and online redo logs at 10h00 am , 12h00,
>04:00 pm.
>Imagine that my server burned at the end of the evening ( 05h00 ).
>Which is the most recent version of my database that can I restore ?
>

you would take the backup from 3am, restore it.

you would roll forward through all of the archived redo logs you have.

if the current online redo logs are available (the ones that were current as of the crash), you will apply them as well -- else you lose their work.

It is 100% possible, and quite normal in fact, to replay all of the ARCHIVED redo log accumulated during the day.

It is not possible however to make use of any ONLINE REDO log files that were backed up -- they are useless. The only thing they could do for you is mess things up.

what happens if you back them up is that you tend to panic during the restore, you restore the datafiles AND online redo log files -- now you have just wiped out the CURRENT ONLINE redo log files. You will LOSE that data. The only thing these backed up files could do is ruin your day.

You backup datafiles.
You backup archives.
You create control file backups (and hopefully you never need them).

You do not backup online redo log, it is never necessary, even with a cold backup (cold backups should be done after a shutdown immediate or normal, redo logs are not necessary to recover).

>If I restore all datafiles saved at 03h00 am and all control files , redo
>log files saved at 04:00pm,
>Why it is not possible replay all the archived redo log saved between
>03h00am and 04h00pm ?
>
>if not, can you tell me what is the best backup strategy for this case of
>damaged ?
>
>Thanks.
>
>Christian.
>
>"Howard J. Rogers" <dba_at_hjrdba.com> a écrit dans le message news:
>3c3d744b$0$1586$afc38c87_at_news.optusnet.com.au...
>> You cannot, ever, in a million years, backup an online redo log. It can't
>> be done, and you'll risk data loss by attempting it (I've seen a junior
>DBA
>> restore everything from a backup set, including a hot-copied online log,
>and
>> as a result over-write a perfectly functioning current redo log with a
>pile
>> of poo).
>>
>> Rogers' first rule of hot backups is simply this: You cannot ever hot
>backup
>> any part of an Oracle database without the end result being internally
>> inconsistent. That goes for online redo logs as well as datafiles or
>> control files.
>>
>> Rogers' second rule of hot backups is that internally inconsistent files
>are
>> not a problem provided Oracle's supplied a mechanism to get them
>internally
>> consistent. The application of redo makes an internally inconsistent data
>> file consistent. But for control files and redo logs, there is no such
>> mechanism (you can't apply redo to either).
>>
>> Now, for control files, Oracle thoughtfully provided the 'alter database
>> backup controlfile to...' command, which uses its own special mechanism to
>> generate either a trace file (and there's no worries about a text file
>being
>> inconsistent) or a guaranteed consistent binary image of the control file.
>>
>> But no such provision was made for taking a hot backup of the current redo
>> log.
>>
>> Therefore, combine rules one and two with the lack of an alternative
>> mechanism and the conclusion is inescapable: hot backups of the current
>redo
>> log will be internally inconsistent, utterly unusable, and no mechanism
>> exists to rectify the situation.... therefore, hot backups of them are a
>> complete waste of time.
>>
>> You can of course perform a cold backup of said redo log, by the simple
>> expedient of shutting the database down.... at which point, you don't
>> actually have a current redo log any more, just a bunch of dead o/s files.
>> There is no problem copying such a closed redo log at all... but it will
>be
>> out of date the second you re-open the database.
>>
>> Visit my site, read the backup and recovery documents there, especially
>the
>> large one under the 'books' link, and you'll see that unless you are
>running
>> in noarchivelog, there is zero usefulness in backing up the current log at
>> all. What's more, doing so risks inadvertent (and unnecessary) loss of
>data
>> when it comes time to recovery. Without doubt, the current redo log is
>the
>> Achilles' heel of the entire Oracle architecture -which is why they
>invented
>> redo log mirroring (or, more accurately) multiplexing so that total loss
>of
>> the current log should be a very rare occurrence -making attempted backups
>> of it even more of a waste of time than they otherwise would be.
>>
>> Regards
>> HJR
>> --
>> ----------------------------------------------
>> Resources for Oracle: http://www.hjrdba.com
>> ===============================
>>
>>
>> "Christian MERCK" <cm_at_sdv.fr> wrote in message
>> news:3c3d4dda$0$14589$4d4eb98e_at_read.news.fr.uu.net...
>> > Hi,
>> >
>> > It is useful to backup regularly the online redo log ?
>> > If yes, how should I do it ? Just an OS copy ?
>> >
>> > Thanks in advance.
>> >
>> > Christian.
>> >
>> >
>>
>>
>
>

--
Thomas Kyte (tkyte@us.oracle.com)             http://asktom.oracle.com/ 
Expert one on one Oracle, programming techniques and solutions for Oracle.
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