Block Media Recovery

You can use Block Media Recovery to recover just corrupted blocks not entire datafile. It has several advantages:

1. It decreases Mean Time To Recover(MTTR). As I mentioned above, you need to recover just corrupted blocks and not all the blocks in datafile.

2. Affected datafile stays online. If you do not use Block Media Recovery you must take datafile offline, which means that affected datafile will be unavailable. During BMR just corrupted blocks are not available.

When you are recovering blocks you must know exact address of the block. Each block is recovered individually.

Also database must be in ARCHIVELOG mode and backup of the database must exist.

I have a table called TEST which is saved in TSTTBS tablespace. I have backuped database, then I’ve opened TSTTBS.DBF datafile by UltraEdit and spoiled block which
belongs to the TEST table. You do not need this steps . I did it for to simulate block corruption.

SQL> select * from a;
select * from a
*
ERROR at line 1:
ORA-01578:ORACLE data block corrupted (file # 5, block # 11)
ORA-01110:data file 5:'C:\ORACLE\PRODUCT\10.2.0\ORADATA\TEST\TSTTBS.DBF'

Error shows that block number 12 is corrupted in datafile 5.

Connect target database by RMAN, and run the following:

RMAN> blockrecover datafile 5 block 11;

It should not take time like 00:00:00 or output shouldn’t be something unusual for example for the firs time I had the following output:

Starting blockrecover at 29-MAR-11
using channel ORA_DISK_1

starting media recovery
media recovery complete, elapsed time: 00:00:00

Finished blockrecover at 29-MAR-11

It was because I have had taken a bad backup. It may also happen when RMAN couldn’t find backup files or you have indicated wrong datafile number or block number.

The followings are restrictions and usage notes of BLOCKRECOVER:

1. The target database must be mounted or open. You do not have to take a datafile offline if you are performing block media recovery on it.
2. You can only perform complete media recovery of individual blocks. Point-in-time recovery of individual data blocks is not supported.
3. You can only perform block media recovery on corrupt blocks.
4. Blocks marked media corrupt are not accessible until recovery completes.
5. You cannot perform block media recovery when using a backup control file.
6. You cannot use proxy backups to perform block media recovery. If the only backups that you have are proxy backups, then you can restore them to a nondefault location on disk, which causes RMAN to view the restored files as datafile copies. You can then use the datafile copies for block media recovery.
7. You must have a full backup of the file containing the corrupt blocks: block media recovery cannot use incremental backups.
8. If RMAN fails to access a specific archived redo log file needed for block media recovery, it performs restore failover, trying all other backups listed in the RMAN repository that are suitable for use in this operation, and only fails if no suitable backup is available. See
9. The datafile header block (block 1) cannot be recovered.
10. You cannot perform block media recovery in NOARCHIVELOG mode.

Let’s check if corrupted block is recovered.

SQL> select * from a;
select * from a
*
ERROR at line 1:
ORA-01578: ORACLE data block corrupted (file # 5, block # 12)
ORA-01110:data file 5:'C:\ORACLE\PRODUCT\10.2.0\ORADATA\TEST\TSTTBS.DBF'

In our example after recovering block  #11 Oracle raised another error indicating that #12 block is corrupted(When multiple blocks are corrupted error is raised one by one). Let’s repeat above steps:

RMAN> blockrecover datafile 5 block 12;

SQL> select * from a;

VC
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Testing Corruption