Redo Log backup

[Former Member]

Dear All;

I take every week an offline + redo log at the weekend.

I already used the offline backup for many things such the quality refresh, but I never used the redo log backup.

Can any one tell what are the redo logs are used for?

Best Regards

~Amal Aloun

[paul_power · Accepted solution]

Hi Amal,

The redo logs would be used especially if it was an online backup, as it would need to rollforward to a consistent point in time. For an offline backup, if you wanted to restore but then rollforward beyond the time of the backup timestamp, the redo logs would be needed.

Regards,

Paul

[Former Member · Accepted solution]

Dear Paul;

Is the concept of the redo logs same as when i take a backup, but then instead of taking a full backup again it would only backup the changes which happened after the last backup. Is this what you mean by roll forward?

Best Regards

~Amal

[paul_power · Accepted solution]

Hi Amal,

Redo logs are mostly used is retoring to a specific point in time:

SAP Note 23070 Backup and Recovery: Basic Concepts

SAP Note 4164 - Incomplete Recovery ("...RECOVER UNTIL

SAP Note 4163 Defective redologs

Backing up only the change since the last full backup would be done via an Incremental Backup

As per note #842240 - FAQ: Backup strategy of large and highly-available
databases:
<====>
   o  Incremental backup

      When you use RMAN, incremental backups can be created. These only
      save the blocks that have been changed since the last full
      backup, which significantly reduces the data volume of the
      backup. The runtime may also be slightly reduced. However, as all
      blocks have to be scanned in each case, the runtime is not
      reduced to the same extent as the data volume. As of Oracle 10g,
      you can also activate Block Change Tracking as described in Note
      964619, which often allows significant improvements in runtime to
      be achieved.

      -  Advantages:

         Only blocks that are actually changed are saved.
         The system also only saves the blocks that contain data in the
         case of an RMAN full backup.
         Runtimes are shorter, especially in conjunction with Block
         Change Tracking.

      -  Disadvantages:

         The restore takes more time and resources, since a full backup
         and an incremental backup have to be restored.

This is a possible incremental backup scenario:
- Sundays: Full backup (level 0) of the database
- Monday to Saturday: Incremental backup (level 1) of the database

See also the following SPA Notes:

964619  RMAN: Incremental backups with Block Change Tracking
842240 FAQ: Backup strategy of large and highly-available
1493614 Parallel incremental backup on disk

Kindly note that incremental is only possible for BRtools 7.20(patch 7)
and above release.

Regards,

Paul

[former_member206552 · Accepted solution]

Hi Amal,

Yes the redo logs are the changes that are made on the system, this is why they are required when doing a point in time recovery(roll forward).

if you would like to roll forward from the time of the last backup then you require the archvie logs from the time of the backup until the time when you want to restore to.

best regards

marius

[Former Member · Accepted solution]

Dear Marius;

Thank you for your answer. you are saying that the use of redo logs are only in case I want to make a recovery from the point where I took the redo logs. so they are only used for a recovery, and in case i need to do a full restore to the system then the offline backup will be enough. please correct me if i am wrong.

Best Regards

~Amal

[karthikeyan_natarajan4 · Accepted solution]

Hi Amal,

Whenever you take backup (whether its online or offline) all committed transactions are backed up.

Redo logs - Information about the data changes will be recorded in the redo logs.

Scenario 1

Hardware failure or power outage - when you restart the system, all the backed up data from data backup will be applied along with that redo log contains information about changes will be applied to have consistent database - same as recovery of most recent consistent state.

Scenario 2. Recovery to point in time - data backup and online redo logs backup will be applied

Scenario 3. Recovery to specific data backup - only data backup will be used.

If you want to restore a system with your offline backup - only data backups will be good, there is no need of redo logs files. Because whenever you take offline backup all the committed transactions will be return to the data backup, all the uncommitted transactions which is basically present in the log area (log segments) will be rolled backward.

regards

kartik

[former_member206552 · Accepted solution]

Hi Amal,

Yes you will be able to recover the database using the offline backup without the redo logs.

offline backup taken at 1 Feb 2013 20:00:00, you can restore to this point using the offline backup, but if you want to restore to 1 Feb 2013 23:00:00 then you will require the redo logs to do the roll forward as they contain all changes mande.

Best Regards

Marius

[Former Member]

Hi Amal,

The redo log file can be used to do On point or On time recovery.

Its like if some one asks you to restore the database having changes upto a certain time. Then you will have to do the On time recovery and here you have to use Database backup as well as the redo log backup generated till that specific time.

Regards,

Shashi

[Former Member]

Ideally If you are taking offline backup on Weekend, You do not need to take redo log backup that time unless you want to restore your database to any point before of your offline backup.

[Reagan]

Can any one tell what are the redo logs are used for?

An inconsistent backup require recovery and for recovery you need archived logs.

Regards

RB