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Dealing with Database Corruption

Dealing with Database Corruption. Objectives. After completing this lesson, you should be able to: Identify the causes of database corruption: Hardware Software Detect database corruption by using: ANALYZE DBVERIFY DB_BLOCK_CHECKING DBMS_REPAIR Repair corruptions by using RMAN.

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Dealing with Database Corruption

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  1. Dealing with Database Corruption

  2. Objectives After completing this lesson, you should be able to: • Identify the causes of database corruption: • Hardware • Software • Detect database corruption by using: • ANALYZE • DBVERIFY • DB_BLOCK_CHECKING • DBMS_REPAIR • Repair corruptions by using RMAN

  3. What Is Block Corruption? • Whenever a block is read or written, a consistency check is performed. • Block version • DBA (data block address) value in cache as compared to the DBA value in the block buffer • Block-checksum, if enabled • A corrupt block is identified as being one of the following: • Media corrupt • Logically (or software) corrupt

  4. Block Corruption Symptoms: ORA-01578 The error ORA-01578: "ORACLE data block corrupted (file # %s, block # %s)": • Is generated when a corrupted data block is found • Always returns the absolute file number and block number • Is returned to the session that issued the query being performed when the corruption was discovered • Appears in the alert.log file

  5. How to Handle Corruption • Check the alert log and operating system log file. • Use available diagnostic tools to find out the type of corruption. • Determine whether the error persists by running checks multiple times. • Recover data from the corrupted object if necessary.

  6. How to Handle Corruption • Resolve any hardware issues: • Memory boards • Disk controllers • Disks • Recover or restore data from the corrupt object if necessary.

  7. Corruption-Related Features

  8. DBVERIFY Utility • Works only on data files; redo log files cannot be checked • Checks block consistency • Can be used while the database is open • Name of the utility program: dbv $ dbv file=/u01/oradata/users01.dbf \ blocksize=8192

  9. Interpreting DBVERIFY Output • A “page” is a block. • If the head and tail do not match, DBVERIFY rereads the block. If they match, an influx block is reported; otherwise, a corruption is signaled. Total Pages Examined : 12800 Total Pages Processed (Data) : 4408 Total Pages Failing (Data) : 0 Total Pages Processed (Index): 1264 . . . Total Pages Marked Corrupt : 4 Total Pages Influx : 0 Highest block SCN : 654836 (0.654836)

  10. ANALYZE Command • Performs a logical block check • Does not mark blocks as soft corrupt; only reports them • Validates index and table entries SQL> ANALYZE TABLE table_name VALIDATE 2 STRUCTURE CASCADE; SQL> ANALYZE INDEX index_name VALIDATE 2 STRUCTURE;

  11. Verifying Block Integrity in Real Time: DB_BLOCK_CHECKING The DB_BLOCK_CHECKING initialization parameter: • Controls the degree of self-consistency checks performed on each block as it is processed • Can prevent memory and data corruption • Can be set by using the ALTERSESSION or ALTER SYSTEMDEFERRED command

  12. Verifying Block Integrity in Real Time: DB_BLOCK_CHECKSUM The DB_BLOCK_CHECKSUM initialization parameter: • Determines whether a checksum is maintained and verified on each block • Can prevent corruption caused by underlying I/O systems a14f

  13. Using EXP to Detect Corruption Conventional export can be used to detect corruption. $ exp hr/hr tables=departments About to export specified tables via Conventional Path ... . . exporting table DEPARTMENTS EXP-00056: ORACLE error 1578 encountered ORA-01578: ORACLE data block corrupted (file # 5, block # 51) ORA-01110: data file 5: '/u01/app/oracle/oradata/orcl/example01.dbf'

  14. Using Flashback for Logical Corruption Flashback Versions Query Flashback Transaction Query DBA Corruptdata found Undo SQL or Flashback Table USER

  15. DBMS_REPAIR Package Available procedures • CHECK_OBJECT • FIX_CORRUPT_BLOCKS • DUMP_ORPHAN_KEYS • REBUILD_FREELISTS • SEGMENT_FIX_STATUS • SKIP_CORRUPT_BLOCKS • ADMIN_TABLES

  16. Using DBMS_REPAIR • Detect and report corruptions. • Evaluate the costs and benefits of DBMS_REPAIR. SET SERVEROUTPUT ON DECLARE num_corrupt INT; BEGIN num_corrupt := 0; DBMS_REPAIR.CHECK_OBJECT ( schema_name => ‘HR', object_name => 'DEPARTMENTS', repair_table_name => 'REPAIR_TABLE', corrupt_count => num_corrupt); END;

  17. Using DBMS_REPAIR • Make objects usable. SET SERVEROUTPUT ON DECLARE num_fix INT; BEGIN num_fix := 0; DBMS_REPAIR.FIX_CORRUPT_BLOCKS ( schema_name => 'HR', object_name => 'DEPARTMENTS', object_type => DBMS_REPAIR.TABLE_OBJECT, repair_table_name => 'REPAIR_TABLE', fix_count => num_fix); END;

  18. Using DBMS_REPAIR • Repair corruptions and rebuild lost data. SET SERVEROUTPUT ON DECLARE num_orphans INT; BEGIN 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 => num_orphans); DBMS_OUTPUT.PUT_LINE('orphan key count: ' || TO_CHAR(num_orphans)); END;

  19. Block Media Recovery (BMR) Block media recovery: • Lowers the mean time to recover (MTTR) • Increases availability during media recovery • The data file remains online during recovery. • Only blocks being recovered are inaccessible. • Is invoked through RMAN via the BLOCKRECOVER command. • Restores individual blocks from available backups • Coordinates with the server to have them recovered

  20. BLOCKRECOVER Command The RMAN BLOCKRECOVER command: • Identifies the backups containing the blocks to recover • Reads the backups and accumulates requested blocks into in-memory buffers • Manages the block media recovery session by reading the archive logs from backup if necessary • Cannot be used for incomplete recovery RMAN> BLOCKRECOVER DATAFILE 6 BLOCK 3;

  21. Examples of Using BLOCKRECOVER • Recovering a group of corrupt blocks • Limiting block media recovery by type of restore • Limiting block media recovery by backup tag • Limiting block media recovery by time, SCN, or log sequence

  22. The RMAN BMR Interface Dynamic views show the current state of corruption. • The V$DATABASE_BLOCK_CORRUPTION view shows the list of currently corrupted database blocks. • The V$BACKUP_CORRUPTION view shows the list of corrupted blocks in data file backups. • The V$COPY_CORRUPTION view shows the list of corrupted blocks in image file copies. RMAN> BLOCKRECOVER CORRUPTION LIST 2> RESTORE UNTIL TIME 'sysdate – 10';

  23. Alternative Actions to Take • Table: The data in the corrupted block is lost. • Drop the table and re-create it, and import data from an export dump. • Use SQL or PL/SQL to pull data out of the table into a newly created table. • Index: Drop and re-create the index.

  24. Summary In this lesson, you should have learned how to: • Identify the causes of database corruption: • Hardware • Software • Detect database corruption by using: • ANALYZE • dbverify • DB_BLOCK_CHECKING • DBMS_REPAIR • Repair corruptions by using RMAN

  25. Practice Overview:Perform Block Media Recovery This practice covers the following topics: • Discovering corruption • Identifying the location of the corruption • Recovering from the corruption by using block media recovery

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