PURPOSE This document provides an example of DBMS_REPAIR as introduced in Oracle 8i. Oracle provides different methods for detecting and correcting data block corruption - DBMS_REPAIR is one option. WARNING: Any corruption that involves the loss of data requires analysis to understand how that data fits into the overall database system. Depending on the nature of the repair, you may lose data and logical inconsistencies can be introduced; therefore you need to carefully weigh the gains and losses associated with using DBMS_REPAIR. SCOPE & APPLICATION This article is intended to assist an experienced DBA working with an Oracle Worldwide Support analyst only. This article does not contain general information regarding the DBMS_REPAIR package, rather it is designed to provide sample code that can be customized by the user (with the assistance of an Oracle support analyst) to address database corruption. The "Detecting and Repairing Data Block Corruption" Chapter of the Oracle8i Administrator's Guide should be read and risk assessment analyzed prior to proceeding. RELATED DOCUMENTS Oracle 8i Administrator's Guide, DBMS_REPAIR Chapter Introduction ============= Note: The DBMS_REPAIR package is used to work with corruption in the transaction layer and the data layer only (software corrupt blocks). Blocks with physical corruption (ex. fractured block) are marked as the block is read into the buffer cache and DBMS_REPAIR ignores all blocks marked corrupt. The only block repair in the initial release of DBMS_REPAIR is to *** mark the block software corrupt ***. DB_BLOCK_CHECKING and DB_BLOCK_CHECKSUM must both be set to FALSE. A backup of the file(s) with corruption should be made before using package. Database Summary =============== A corrupt block exists in table T1. SQL> desc t1 Name Null? Type ----------------------------------------- -------- ---------------------------- COL1 NOT NULL NUMBER(38) COL2 CHAR(512) SQL> analyze table t1 validate structure; analyze table t1 validate structure * ERROR at line 1: ORA-01498: block check failure - see trace file ---> Note: In the trace file produced from the ANALYZE, it can be determined --- that the corrupt block contains 3 rows of data (nrows = 3). --- The leading lines of the trace file follows: Dump file /export/home/oracle/product/8.1.5/admin/V815/udump/v815_ora_2835.trc Oracle8 Enterprise Edition Release 8.1.5.0.0 - Beta With the Partitioning option *** 1998.12.16.15.53.02.000 *** SESSION ID:(7.6) 1998.12.16.15.53.02.000 kdbchk: row locked by non-existent transaction table=0 slot=0 lockid=32 ktbbhitc=1 Block header dump: 0x01800003 Object id on Block? Y seg/obj: 0xb6d csc: 0x00.1cf5f itc: 1 flg: - typ: 1 - DATA fsl: 0 fnx: 0x0 ver: 0x01 Itl Xid Uba Flag Lck Scn/Fsc 0x01 xid: 0x0002.011.00000121 uba: 0x008018fb.0345.0d --U- 3 fsc 0x0000.0001cf60 data_block_dump =============== tsiz: 0x7b8 hsiz: 0x18 pbl: 0x28088044 bdba: 0x01800003 flag=----------- ntab=1 nrow=3 frre=-1 fsbo=0x18 fseo=0x19d avsp=0x185 tosp=0x185 0xe:pti[0] nrow=3 offs=0 0x12:pri[0] offs=0x5ff 0x14:pri[1] offs=0x3a6 0x16:pri[2] offs=0x19d block_row_dump: [... remainder of file not included] end_of_block_dump DBMS_REPAIR.ADMIN_TABLES (repair and orphan key ================================================ ADMIN_TABLES provides administrative functions for repair and orphan key tables. SQL> @adminCreate SQL> connect sys/change_on_install Connected. SQL> SQL> -- Repair Table SQL> SQL> declare 2 begin 3 -- Create repair table 4 dbms_repair.admin_tables ( 5 -- table_name => 'REPAIR_TABLE', 6 table_type => dbms_repair.repair_table, 7 action => dbms_repair.create_action, 8 tablespace => 'USERS'); -- default TS of SYS if not specified 9 end; 10 / PL/SQL procedure successfully completed. SQL> select owner, object_name, object_type 2 from dba_objects 3 where object_name like '%REPAIR_TABLE'; OWNER OBJECT_NAME OBJECT_TYPE ------------------------------------------------------------------ SYS DBA_REPAIR_TABLE VIEW SYS REPAIR_TABLE TABLE SQL> SQL> -- Orphan Key Table SQL> SQL> declare 2 begin 3 -- Create orphan key table 4 dbms_repair.admin_tables ( 5 table_type => dbms_repair.orphan_table, 6 action => dbms_repair.create_action, 7 tablespace => 'USERS'); -- default TS of SYS if not specified 8 end; 9 / PL/SQL procedure successfully completed. SQL> select owner, object_name, object_type 2 from dba_objects 3 where object_name like '%ORPHAN_KEY_TABLE'; OWNER OBJECT_NAME OBJECT_TYPE ------------------------------------------------------------------ SYS DBA_ORPHAN_KEY_TABLE VIEW SYS ORPHAN_KEY_TABLE TABLE DBMS_REPAIR.CHECK_OBJECT ========================= CHECK_OBJECT procedure checks the specified object and populates the repair table with information about corruption and repair directive(s). Validation consists of block checking all blocks in the object. All blocks previously marked corrupt will be skipped. Note: In the initial release of DBMS_REPAIR the only repair is to mark the block as software corrupt. SQL> @checkObject SQL> set serveroutput on SQL> SQL> declare 2 rpr_count int; 3 begin 4 rpr_count := 0; 5 dbms_repair.check_object ( 6 schema_name => 'SYSTEM', 7 object_name => 'T1', 8 repair_table_name => 'REPAIR_TABLE', 9 corrupt_count => rpr_count); 10 dbms_output.put_line('repair count: ' || to_char(rpr_count)); 11 end; 12 / repair count: 1 PL/SQL procedure successfully completed. SQL> desc repair_table Name Null? Type ----------------------------------------- -------- ---------------------------- OBJECT_ID NOT NULL NUMBER TABLESPACE_ID NOT NULL NUMBER RELATIVE_FILE_ID NOT NULL NUMBER BLOCK_ID NOT NULL NUMBER CORRUPT_TYPE NOT NULL NUMBER SCHEMA_NAME NOT NULL VARCHAR2(30) OBJECT_NAME NOT NULL VARCHAR2(30) BASEOBJECT_NAME VARCHAR2(30) PARTITION_NAME VARCHAR2(30) CORRUPT_DESCRIPTION VARCHAR2(2000) REPAIR_DESCRIPTION VARCHAR2(200) MARKED_CORRUPT NOT NULL VARCHAR2(10) CHECK_TIMESTAMP NOT NULL DATE FIX_TIMESTAMP DATE REFORMAT_TIMESTAMP DATE SQL> select object_name, block_id, corrupt_type, marked_corrupt, 2 corrupt_description, repair_description 3 from repair_table; OBJECT_NAME BLOCK_ID CORRUPT_TYPE MARKED_COR ------------------------------ ---------- ------------ ---------- CORRUPT_DESCRIPTION -------------------------------------------------------------------------------- REPAIR_DESCRIPTION -------------------------------------------------------------------------------- T1 3 1 FALSE kdbchk: row locked by non-existent transaction table=0 slot=0 lockid=32 ktbbhitc=1 mark block software corrupt Data Extraction =============== The repair table indicates that block 3 of file 6 is corrupt - but remember that this block has not yet been marked as corrupt, therefore now is the time to extract any meaningful data. After the block is marked corrupt, the entire block must be skipped. 1. Determine the number of rows in the block from ALTER SYSTEM DUMP (nrows = 3). 2. Query the corrupt object and extract as much information as possible. SQL> -- The following query can be used to salvage data from a corrupt block. SQL> -- Creating a temporary table facilitates data insertion. SQL> create table temp_t1 as 2 select * from system.t1 3 where dbms_rowid.rowid_block_number(rowid) = 3 4 and dbms_rowid.rowid_to_absolute_fno (rowid, 'SYSTEM','T1') = 6; Table created. SQL> select col1 from temp_t1; COL1 ---------- 2 3 DBMS_REPAIR.FIX_CORRUPT_BLOCKS (ORA-1578) ============================================ FIX_CORRUPT_BLOCKS procedure fixes the corrupt blocks in the specified objects based on information in the repair table. After the block has been marked as corrupt, an ORA-1578 results when a full table scan is performed. SQL> declare 2 fix_count int; 3 begin 4 fix_count := 0; 5 dbms_repair.fix_corrupt_blocks ( 6 schema_name => 'SYSTEM', 7 object_name => 'T1', 8 object_type => dbms_repair.table_object, 9 repair_table_name => 'REPAIR_TABLE', 10 fix_count => fix_count); 11 dbms_output.put_line('fix count: ' || to_char(fix_count)); 12 end; 13 / fix count: 1 PL/SQL procedure successfully completed. SQL> select object_name, block_id, marked_corrupt 2 from repair_table; OBJECT_NAME BLOCK_ID MARKED_COR ------------------------------ ---------- ---------- T1 3 TRUE SQL> select * from system.t1; select * from system.t1 * ERROR at line 1: ORA-01578: ORACLE data block corrupted (file # 6, block # 3) ORA-01110: data file 6: '/tmp/ts_corrupt.dbf' DBMS_REPAIR.DUMP_ORPHAN_KEYS ============================== DUMP_ORPHAN_KEYS reports on index entries that point to rows in corrupt data blocks. SQL> select index_name from dba_indexes 2 where table_name in (select distinct object_name from repair_table); INDEX_NAME ------------------------------ T1_PK SQL> @dumpOrphanKeys SQL> set serveroutput on SQL> SQL> declare 2 key_count int; 3 begin 4 key_count := 0; 5 dbms_repair.dump_orphan_keys ( 6 schema_name => 'SYSTEM', 7 object_name => 'T1_PK', 8 object_type => dbms_repair.index_object, 9 repair_table_name => 'REPAIR_TABLE', 10 orphan_table_name => 'ORPHAN_KEY_TABLE', 11 key_count => key_count); 12 dbms_output.put_line('orphan key count: ' || to_char(key_count)); 13 end; 14 / orphan key count: 3 PL/SQL procedure successfully completed. SQL> desc orphan_key_table Name Null? Type ----------------------------------------- -------- ---------------------------- SCHEMA_NAME NOT NULL VARCHAR2(30) INDEX_NAME NOT NULL VARCHAR2(30) IPART_NAME VARCHAR2(30) INDEX_ID NOT NULL NUMBER TABLE_NAME NOT NULL VARCHAR2(30) PART_NAME VARCHAR2(30) TABLE_ID NOT NULL NUMBER KEYROWID NOT NULL ROWID KEY NOT NULL ROWID DUMP_TIMESTAMP NOT NULL DATE SQL> select index_name, count(*) from orphan_key_table 2 group by index_name; INDEX_NAME COUNT(*) ------------------------------ ---------- T1_PK 3 Note: Index entry in the orphan key table implies that the index should be rebuilt to guarantee the a table probe and an index probe return the same result set. DBMS_REPAIR.SKIP_CORRUPT_BLOCKS =============================== SKIP_CORRUPT_BLOCKS enables/disables the skipping of corrupt blocks during index and table scans of a specified object. Note: If an index and table are out of sync, then a SET TRANSACTION READ ONLY transaction may be inconsistent in situations where one query probes only the index and then a subsequent query probes both the index and the table. If the table block is marked corrupt, then the two queries will return different results. Suggestion: If SKIP_CORRUPT_BLOCKS is enabled, then rebuild any indexes identified in the orphan key table (or all index associated with object if DUMP_ORPHAN_KEYS was omitted). SQL> @skipCorruptBlocks SQL> declare 2 begin 3 dbms_repair.skip_corrupt_blocks ( 4 schema_name => 'SYSTEM', 5 object_name => 'T1', 6 object_type => dbms_repair.table_object, 7 flags => dbms_repair.skip_flag); 8 end; 9 / PL/SQL procedure successfully completed. SQL> select table_name, skip_corrupt from dba_tables 2 where table_name = 'T1'; TABLE_NAME SKIP_COR ------------------------------ -------- T1 ENABLED SQL> -- rows in corrupt block skipped, no errors on full table scan SQL> select * from system.t1; COL1 COL2 -------------------------------------------------------------------------------- 4 dddd 5 eeee --> Notice the pk index has not yet been corrected. SQL> insert into system.t1 values (1,'aaaa'); insert into system.t1 values (1,'aaaa') * SQL> select * from system.t1 where col1 = 1; no rows selected DBMS_REPAIR.REBUILD_FREELISTS =============================== REBUILD_FREELISTS rebuilds freelists for the specified object. SQL> declare 2 begin 3 dbms_repair.rebuild_freelists ( 4 schema_name => 'SYSTEM', 5 object_name => 'T1', 6 object_type => dbms_repair.table_object); 7 end; 8 / PL/SQL procedure successfully completed. Rebuild Index ============= Note: Every index identified in the orphan key table should be rebuilt to ensure consistent results. SQL> alter index system.t1_pk rebuild online; Index altered. SQL> insert into system.t1 values (1, 'aaaa'); 1 row created. SQL> select * from system.t1; COL1 COL2 -------------------------------------------------------------------------------- 4 dddd 5 eeee 1 aaaa Note - The above insert statement was used to provide a simple example. This is the perfect world - we know the data that was lost. The temporary table (temp_t1) should also be used to include all rows extracted from the corrupt block. Conclusion ========== At this point the table T1 is available but data loss was incurred. In general, data loss must be seriously considered before using the DBMS_REPAIR package for mining the index segment and/or table block dumps is very complicated and logical inconsistencies may be introduced. In the initial release, the only repair affected by DBMS_REPAIR is to mark the block as software corrupt. <<End of Article>
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