7 Dealing with Database Corruption Copyright 2006 Oracle

Objectives After completing this lesson, you should be able to: • Identify the causes of database corruption: – Hardware – Software • Detect database corruption by using: – – • 7 -2 ANALYZE DBVERIFY DB_BLOCK_CHECKING DBMS_REPAIR Repair corruptions by using RMAN Copyright © 2006, Oracle. All rights reserved.

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 7 -3 Copyright © 2006, Oracle. All rights reserved.

Block Corruption Symptoms: ORA-01578 The error ORA-01578: "ORACLE data block corrupted (file # %s, block # %s)": • • 7 -4 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 Copyright © 2006, Oracle. All rights reserved.

How to Handle Corruption • • 7 -5 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. Copyright © 2006, Oracle. All rights reserved.

How to Handle Corruption • Resolve any hardware issues: – Memory boards – Disk controllers – Disks • 7 -6 Recover or restore data from the corrupt object if necessary. Copyright © 2006, Oracle. All rights reserved.

Corruption-Related Features Feature 7 -7 Corruption Detected Repairs Corruption DBVERIFY Physical FALSE ANALYZE Logical FALSE DB_BLOCK_CHECKING Logical FALSE DB_BLOCK_CHECKSUM Physical FALSE exp Physical FALSE Flashback Logical TRUE DBMS_REPAIR Logical TRUE Block media recovery None TRUE Copyright © 2006, Oracle. All rights reserved.

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=/u 01/oradata/users 01. dbf blocksize=8192 7 -8 Copyright © 2006, Oracle. All rights reserved.

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 : Total Pages Processed (Data) : Total Pages Failing (Data) : Total Pages Processed (Index): . . . Total Pages Marked Corrupt : Total Pages Influx : Highest block SCN : 7 -9 12800 4408 0 1264 4 0 654836 (0. 654836) Copyright © 2006, Oracle. All rights reserved.

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; 7 -11 Copyright © 2006, Oracle. All rights reserved.

Verifying Block Integrity in Real Time: DB_BLOCK_CHECKING The DB_BLOCK_CHECKING initialization parameter: • • • 7 -12 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 ALTER SESSION or ALTER SYSTEM DEFERRED command Copyright © 2006, Oracle. All rights reserved.

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 a 14 f 7 -13 Copyright © 2006, Oracle. All rights reserved.

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: '/u 01/app/oracle/oradata/orcl/example 01. dbf' 7 -14 Copyright © 2006, Oracle. All rights reserved.

Using Flashback for Logical Corruption Flashback Versions Query Flashback Transaction Query DBA Corrupt data

DBMS_REPAIR Package Available procedures • CHECK_OBJECT • FIX_CORRUPT_BLOCKS • DUMP_ORPHAN_KEYS • REBUILD_FREELISTS • SEGMENT_FIX_STATUS • SKIP_CORRUPT_BLOCKS • ADMIN_TABLES 7 -16 Copyright © 2006, Oracle. All rights reserved.

Using DBMS_REPAIR 1. Detect and report corruptions. 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; 2. Evaluate the costs and benefits of DBMS_REPAIR. 7 -17 Copyright © 2006, Oracle. All rights reserved.

Using DBMS_REPAIR 3. 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; 7 -19 Copyright © 2006, Oracle. All rights reserved.

Using DBMS_REPAIR 4. 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; 7 -20 Copyright © 2006, Oracle. All rights reserved.

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 7 -21 Copyright © 2006, Oracle. All rights reserved.

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; 7 -22 Copyright © 2006, Oracle. All rights reserved.

Examples of Using BLOCKRECOVER • • 7 -23 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 Copyright © 2006, Oracle. All rights reserved.

The RMAN BMR Interface Dynamic views show the current state of corruption. • The V$DATABASE_BLOCK_CORRUPTION view shows the list of currently corrupted database blocks. RMAN> BLOCKRECOVER CORRUPTION LIST 2> RESTORE UNTIL TIME 'sysdate – 10'; • • 7 -25 The V$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. Copyright © 2006, Oracle. All rights reserved.

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. • 7 -26 Index: Drop and re-create the index. Copyright © 2006, Oracle. All rights reserved.

Summary In this lesson, you should have learned how to: • Identify the causes of database corruption: – Hardware – Software • Detect database corruption by using: – – • 7 -27 ANALYZE dbverify DB_BLOCK_CHECKING DBMS_REPAIR Repair corruptions by using RMAN Copyright © 2006, Oracle. All rights reserved.

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 7 -28 Copyright © 2006, Oracle. All rights reserved.