Detect And Correct Corruption

Oracle provides a number of methods to detect and repair corruption within datafiles:

DBVerify
ANALYZE .. VALIDATE STRUCTURE
DB_BLOCK_CHECKING.
DBMS_REPAIR.
Other Repair Methods.

DBVerify

DBVerify is an external utility that allows validation of offline datafiles. In addition to offline datafiles it can be used to check the validity of backup datafiles:

C:>dbv file=C:\Oracle\oradata\TSH1\system01.dbf feedback=100 blocksize=4096

ANALYZE .. VALIDATE STRUCTURE

The ANALYZE command can be used to verify each data block in the analyzed object. If any corruption is detected rows are added to the INVALID_ROWS table:

-- Create the INVALID_ROWS table
SQL> @C:\Oracle\901\rdbms\admin\UTLVALID.SQL

-- Validate the table structure.
SQL> ANALYZE TABLE scott.emp VALIDATE STRUCTURE;

-- Validate the table structure along with all it's indexes.
SQL> ANALYZE TABLE scott.emp VALIDATE STRUCTURE CASCADE;

-- Validate the index structure.
SQL> ANALYZE INDEX scott.pk_emp VALIDATE STRUCTURE;

DB_BLOCK_CHECKING

When the DB_BLOCK_CHECKING parameter is set to TRUE Oracle performs a walk through of the data in the block to check it is self-consistent. Unfortunately block checking can add between 1 and 10% overhead to the server. Oracle recommend setting this parameter to TRUE if the overhead is acceptable.

DBMS_REPAIR

Unlike the previous methods dicussed, the DBMS_REPAIR package allows you to detect and repair corruption. The process requires two administration tables to hold a list of corrupt blocks and index keys pointing to those blocks. These are created as follows:

BEGIN
  DBMS_REPAIR.admin_tables (
    table_name => 'REPAIR_TABLE',
    table_type => DBMS_REPAIR.repair_table,
    action     => DBMS_REPAIR.create_action,
    tablespace => 'USERS');

  DBMS_REPAIR.admin_tables (
    table_name => 'ORPHAN_KEY_TABLE',
    table_type => DBMS_REPAIR.orphan_table,
    action     => DBMS_REPAIR.create_action,
    tablespace => 'USERS');
END;
/

With the administration tables built we are able to check the table of interest using the CHECK_OBJECT procedure:

SET SERVEROUTPUT ON
DECLARE
  v_num_corrupt INT;
BEGIN
  v_num_corrupt := 0;
  DBMS_REPAIR.check_object (
    schema_name       => 'SCOTT',
    object_name       => 'DEPT',
    repair_table_name => 'REPAIR_TABLE',
    corrupt_count     =>  v_num_corrupt);
  DBMS_OUTPUT.put_line('number corrupt: ' || TO_CHAR (v_num_corrupt));
END;
/

Assuming the number of corrupt blocks is greater than 0 the CORRUPTION_DESCRIPTION and the REPAIR_DESCRIPTION columns of the REPAIR_TABLE can be used to get more information about the corruption.

At this point the currupt blocks have been detected, but are not marked as corrupt. The FIX_CORRUPT_BLOCKS procedure can be used to mark the blocks as corrupt, allowing them to be skipped by DML once the table is in the correct mode:

SET SERVEROUTPUT ON
DECLARE
  v_num_fix INT;
BEGIN
  v_num_fix := 0;
  DBMS_REPAIR.fix_corrupt_blocks (
    schema_name       => 'SCOTT',
    object_name       => 'DEPT',
    object_type       => Dbms_Repair.table_object,
    repair_table_name => 'REPAIR_TABLE',
    fix_count         => v_num_fix);
  DBMS_OUTPUT.put_line('num fix: ' || TO_CHAR(v_num_fix));
END;
/

Once the corrupt table blocks have been located and marked all indexes must be checked to see if any of their key entries point to a corrupt block. This is done using the DUMP_ORPHAN_KEYS procedure:

SET SERVEROUTPUT ON
DECLARE
  v_num_orphans INT;
BEGIN
  v_num_orphans := 0;
  DBMS_REPAIR.dump_orphan_keys (
    schema_name       => 'SCOTT',
    object_name       => 'PK_DEPT',
    object_type       => DBMS_REPAIR.index_object,
    repair_table_name => 'REPAIR_TABLE',
    orphan_table_name => 'ORPHAN_KEY_TABLE',
    key_count         => v_num_orphans);
  DBMS_OUTPUT.put_line('orphan key count: ' || TO_CHAR(v_num_orphans));
END;

/

If the orphan key count is greater than 0 the index should be rebuilt.

The process of marking the table block as corrupt automatically removes it from the freelists. This can prevent freelist access to all blocks following the corrupt block. To correct this the freelists must be rebuilt using the REBUILD_FREELISTS procedure:

BEGIN
  DBMS_REPAIR.rebuild_freelists (
    schema_name => 'SCOTT',
    object_name => 'DEPT',
    object_type => DBMS_REPAIR.table_object);
END;
/

The final step in the process is to make sure all DML statements ignore the data blocks marked as corrupt. This is done using the SKIP_CORRUPT_BLOCKS procedure:

BEGIN
  DBMS_REPAIR.skip_corrupt_blocks (
    schema_name => 'SCOTT',
    object_name => 'DEPT',
    object_type => DBMS_REPAIR.table_object,
    flags       => DBMS_REPAIR.skip_flag);
END;
/

The SKIP_CORRUPT column in the DBA_TABLES view indicates if this action has been successful.

At this point the table can be used again but you will have to take steps to correct any data loss associated with the missing blocks.

Other Repair Methods

Other methods to repair corruption include:

Full database recovery.
Individual datafile recovery.
Block media recovery (BMR), available in Oracle9i when using RMAN.
Recreate the table using the CREATE TABLE .. AS SELECT command, taking care to avoid the corrupt blocks by retricting the where clause of the query.
Drop the table and restore it from a previous export. This may require some manual effort to replace missing data.

For more information see:

DBMS_REPAIR

Hope this helps. Regards Tim...

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