backup question

[SANJAY_G]

Allow me to disagree here jmodic,

*have to* is conditional of course but it COULD be a possible scenario.
e.g.
The cold backup is the only backup available
or
the hotbackup taken was much earlier than the coldbackup and there was not much (or none) activity after coldbackup and before db crash.
or
the "estimated recovery time" from last available hotbackup is much longer than that from last available coldbackup.
or
The business needs databse immediately available at cost of losing some data.

So in my view it is a possible recovery scenario.

And AFAIK Oracle recommends to take hot or coldbackup immediately after RESETLOGS because the previous backups can not be used to recover upto a point in time after RESETLOGS. (Yes, there is also a complicated procedure for "recovery across resetlogs")

And I think there may be many other solutions to recover BUT wouldn't it be just nice to backup redo logs with coldbackups (which doesn't take much time and space) to avoid all these hassles and recover cleanly from a coldbackup in the scenarios i mentioned above.

Please correct me if I am wrong.

Sanjay

[SANJAY_G]

Originally posted by pando:
Sanjay I guess you have not used RMAN for cold backups. RMAN does not backup online redo logs whatsoever, not in cold backups or hot backups.

I agree pando that RMAN doesn't backup redo logs whatsoever but I guess (not tested yet) it will recreate the redo logs in case you lose them and restore from a RMAN coldbackup and you need not use RESETLOGS.

But in case of traditional OS coldbackup I think the redo logs should be backed up.

Sanjay

[pando]

well when you open resetlog the redo logs are recreated automatically!

and that´s what RMAN does

[SANJAY_G]

Originally posted by Pando:
well when you open resetlog the redo logs are recreated automatically!

and that´s what RMAN does

If RMAN does RESETLOGS while restoring from coldbackup do we need to issue "reset database" ?
And will it create a new incarnation of the databse in the recovery catalog ?

Just curious, because my understanding is, in RMAN you have to issue "reset databse" after RESETLOGS.

Sanjay

[jmodic]

Originally posted by SANJAY_G
And I think there may be many other solutions to recover BUT wouldn't it be just nice to backup redo logs with coldbackups (which doesn't take much time and space) to avoid all these hassles and recover cleanly from a coldbackup in the scenarios i mentioned above.

Sanyaj,

The point is not in how much space or time it takes to backup redologs as a part of a cold backup, the main point is:

- it could be dangerous to have them with the cold backup
(of course, if DBA knows exectly what he is doing and if we can count on that he/she never makes mistakes, not even when he/she is under extreme pressure which is tipically during recovery occasions, then backuped redo logs mean no harm)

- backed online redo logs are never ever needed for recovery purposes in any recovery scenario

That's why even Oracle manuals suggest not to backup redo logs, not even with cold backups.

And if I might add my personal attitude to this "never include redo logs in cold backup": For occasional cold backups that have very specific purposes (for example for database cloning) and are tipically not stored on any tape for future use, I simply copy redo logs as well. But if I would have to include cold backup in any *periodical* backup scenario (cold backup once a day or once a week or ....) on archlog mode database, I would NEVER include copying redo logs in that scenario. NEVER.

[pando]

please refer Note:61801.1 on Metalink

[marist89]

Originally posted by jmodic
- it could be dangerous to have them with the cold backup
(of course, if DBA knows exectly what he is doing and if we can count on that he/she never makes mistakes, not even when he/she is under extreme pressure which is tipically during recovery occasions, then backuped redo logs mean no harm)
[/B]

Although I don't subscribe to what apearantly is the conventional wisdom about backing up redo logs during a cold backup, I can't really see how it would be dangerous.

[opinion]
I don't use cold backups, so maybe I'm just talking from inexperience. Restore/recovery from a cold backup in noarchivelog mode is a simple procedure; restore all your files and bring up the database. Sure, if the redo logs are not there you can recreate them. In fact, if the redo logs are NOT present, I think it makes the restore MORE difficult.
[/opinion]

[jmodic]

Originally posted by marist89
Restore/recovery from a cold backup in noarchivelog mode is a simple procedure; restore all your files and bring up the database.

Jeff, that is exactly where all the danger is hidden!!! Because it is so simple and straightforward! Even very inexperienced DBA knows what he has to do with cold backup when database crashes - he simply hes to restore *everything* from backup and that's it, that's all it has to be done.

Now change just a few things in the scenario and even not so inexperience DBA could make an unrecoverable mistake. Imagine the following:

Database is running in archivelog mode. Hot backup is taken every night except on Sunday night, when it is shut down so that cold backup can be taken once a week. (Now, don't ask me for the ratio behind this backup scenario, but it seems that is not so very unusual for the archivelog database to be backed up periodicaly with cold backup - yes, I know how stupid this actualy is!). On Monday afternoon database crashes bacause of a disk failure and DBA is urged to make it available as soon as posible without any data loss (original redo logs and todays archlogs are intact, so there should be no data loss).

The most recent backup is the cold backup from last night and DBA knows exactly what he has to do with cold backup. After all, everybody knows that - simply restore everything from the tape and that's it. After restore is complete, he begins recover database to apply archlogs. And after all archlogs are applied - bummer! He realised that he has overwritten online redo logs with the ones from cold backup! So he has to open database with resetlogs anyway, but more importantly - he lost all the transactions from online redo logs! He has lost them forever, unrecoverable, because redo logs were overwritten by unusable ones from cold backup.

This is not so unusual and not totaly hypotetical scenario - it has happened soooo many times that Orcale decided to include the recomendation of not to backup online redo logs in any kind of backup plan in their manuals in recent years!

And I emphasize again: There is not a single recovery scenario anyone could immagine in which backup redo logs would be needed for recovery purposes! Not a single one! So if you don't need them for recovery, why would they need to be included in any backup?

[Edited by jmodic on 10-04-2002 at 02:03 PM]

[np70]

Originally posted by jmodic
If you loose everything and have to restore from cold backup (BTW, why would you *have to* restore from cold backup, you could recover from hot backup as well), why would your previous hot backups be invalidated? They are still perfectly valid, no matter if you had to open your database with RESETLOGS. You still can recover to any point in time, both prior to resetlogs (in which case your starting backup would be any hot backup taken prior to RESETLOGS) and after it (in which case you would use any backup taken after the RESETLOGS).

[Edited by jmodic on 10-04-2002 at 05:13 AM]

Jurij,
AFAIK Once you open database with RESETLOGS, all your previous backups are invalid because SCN gets reset. Thats why Oracle recommends to take cold backup after RESETLOGS.

please let me know if i missed something..

np70

[marist89]

Originally posted by np70

Originally posted by jmodic
If you loose everything and have to restore from cold backup (BTW, why would you *have to* restore from cold backup, you could recover from hot backup as well), why would your previous hot backups be invalidated? They are still perfectly valid, no matter if you had to open your database with RESETLOGS. You still can recover to any point in time, both prior to resetlogs (in which case your starting backup would be any hot backup taken prior to RESETLOGS) and after it (in which case you would use any backup taken after the RESETLOGS).

[Edited by jmodic on 10-04-2002 at 05:13 AM]

Jurij,
AFAIK Once you open database with RESETLOGS, all your previous backups are invalid because SCN gets reset. Thats why Oracle recommends to take cold backup after RESETLOGS.

please let me know if i missed something..

np70

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