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Oracle® Database Backup and Recovery Advanced User's Guide
10g Release 2 (10.2)

Part Number B14191-02
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Block Media Recovery with RMAN

Although datafile media recovery is the principal form of recovery, you can also use the RMAN BLOCKRECOVER command to perform block media recovery. Block media recovery recovers an individual corrupt datablock or set of datablocks within a datafile. In cases when a small number of blocks require media recovery, you can selectively restore and recover damaged blocks rather than whole datafiles.

Block media recovery provides several advantages over datafile media recovery. For example, block media recovery

Note these restrictions of block media recovery:

When to Use Block Media Recovery

Block media recovery is not intended for cases where the extent of data loss or corruption is unknown and the entire datafile requires recovery. In such cases, datafile media recovery is the best solution. Block media recovery is not a replacement for traditional datafile media recovery, but a supplement to it.

In most cases, the database marks a block as media corrupt, invalidates the block in the instances (or all enabled instances in a Real Application Clusters configuration), and then writes it to disk when the corruption is first encountered. No subsequent read of the block will be successful until the block is recovered. You can only perform block recovery on blocks that are marked corrupt. This corrupt status effectively takes the block offline in all database instances and prevents user access during recovery.

Block media recovery is most useful for data losses that affect specific blocks. Block-level data loss usually results from intermittent, random I/O errors that do not cause widespread data loss, as well as memory corruptions that get written to disk. Typically, these types of block corruption are reported in the following locations:

  • Error messages in standard output

  • The alert log

  • User trace files

  • Results of the SQL commands ANALYZE TABLE and ANALYZE INDEX

  • Results of the DBVERIFY utility

  • Third-party media management output

For example, you may discover the following messages in a user trace file:

ORA-01578: ORACLE data block corrupted (file # 7, block # 3)
ORA-01110: data file 7: '/oracle/oradata/trgt/tools01.dbf'
ORA-01578: ORACLE data block corrupted (file # 2, block # 235)
ORA-01110: data file 2: '/oracle/oradata/trgt/undotbs01.dbf'

You can then specify the corrupt blocks in the BLOCKRECOVER command as follows:

BLOCKRECOVER
  DATAFILE 7 BLOCK 3 
  DATAFILE 2 BLOCK 235;

Block Media Recovery When Redo Is Missing

Like datafile media recovery, block media recovery cannot generally survive a missing or inaccessible archived log (although it will attempt restore failover when looking for usable copies of archived redo log files, as described in "Restore Failover"). Nevertheless, block media recovery can survive gaps in the redo stream if the missing or corrupt redo records do not affect the blocks being recovered. Whereas datafile recovery requires an unbroken series of redo changes from the beginning of recovery to the end, block media recovery only requires an unbroken set of redo changes for the blocks being recovered.

Note:

Each block is recovered independently during block media recovery, so recovery may be successful for a subset of blocks.

When RMAN first detects missing or corrupt redo records during block media recovery, it does not immediately signal an error because the block undergoing recovery may become a newed block later in the redo stream. When a block is newed all previous redo for that block becomes irrelevant because the redo applies to an old incarnation of the block. For example, the database can new a block when users delete all the rows recorded in the block or drop a table.

Assume that media recovery is performed on block 13 as depicted in Figure 3-2.

Figure 3-2 Performing RMAN Media Recovery

Description of Figure 3-2 follows
Description of "Figure 3-2 Performing RMAN Media Recovery"

After block recovery begins, RMAN discovers that change 120 is missing. RMAN does not terminate recovery in the hope that block 13 will be newed later in the redo stream. Assume that in change 140 a user drops the table EMPLOYEE stored in block 13. At this point, the database formats block 13 as a new block. Because the redo for block 13 in change 120 related to the EMPLOYEE table, and the EMPLOYEE table was dropped in change 140, RMAN can skip this missing change and apply the redo between changes 140 and 160.