Using DBMS_REPAIR to Repair Data Block Corruption in 10G Release 2
Using DBMS_REPAIR to Repair Data Block Corruption
This chapter explains how to use the
DBMS_REPAIR PL/SQL package to repair data block corruption in database schema objects. It contains the following topics:
- Options for Repairing Data Block Corruption
- About the DBMS_REPAIR Package
- Using the DBMS_REPAIR Package
- DBMS_REPAIR Examples
If you are not familiar with the
DBMS_REPAIRpackage, then it is recommended that you work with an Oracle Support Services analyst when performing any of the repair procedures included in this package.
Options for Repairing Data Block Corruption
Oracle Database provides different methods for detecting and correcting data block corruption. One method of correction is to drop and re-create an object after the corruption is detected. However, this is not always possible or desirable. If data block corruption is limited to a subset of rows, then another option is to rebuild the table by selecting all data except for the corrupt rows.
Another way to manage data block corruption is to use the
DBMS_REPAIR package. You can use
DBMS_REPAIR to detect and repair corrupt blocks in tables and indexes. You can continue to use objects while you attempt to rebuild or repair them.
Any corruption that involves the loss of data requires analysis and understanding of how that data fits into the overall database system. Depending on the nature of the repair, you might lose data, and logical inconsistencies can be introduced. You must determine whether the repair approach provided by this package is the appropriate tool for each specific corruption problem.
This section describes the procedures contained in the
DBMS_REPAIR package and notes some limitations and restrictions on their use.
These procedures are further described, with examples of their use, in “DBMS_REPAIR Examples”.
Limitations and Restrictions
- Tables with LOB datatypes, nested tables, and varrays are supported, but the out of line columns are ignored. xx what are out-of-line columns?
- Clusters are supported in the
REBUILD_FREELISTSprocedures, but not in the
- Index-organized tables and LOB indexes are not supported.
DUMP_ORPHAN_KEYSprocedure does not operate on bitmap indexes or function-based indexes.
DUMP_ORPHAN_KEYSprocedure processes keys that are no more than 3,950 bytes long.
The following approach is recommended when considering
DBMS_REPAIR for addressing data block corruption:
- Task 1: Detect and Report Corruptions
- Task 2: Evaluate the Costs and Benefits of Using DBMS_REPAIR
- Task 3: Make Objects Usable
- Task 4: Repair Corruptions and Rebuild Lost Data
Task 1: Detect and Report Corruptions
The first task is the detection and reporting of corruptions. Reporting not only indicates what is wrong with a block, but also identifies the associated repair directive. There are several ways to detect corruptions. Table 21-1 describes the different detection methodologies.
DBMS_REPAIR: Using the CHECK_OBJECT and ADMIN_TABLES Procedures
CHECK_OBJECT procedure checks and reports block corruptions for a specified object. Similar to the
ANALYZE...VALIDATE STRUCTURE statement for indexes and tables, block checking is performed for index and data blocks.
Not only does
CHECK_OBJECT report corruptions, but it also identifies any fixes that would occur if
FIX_CORRUPT_BLOCKS is subsequently run on the object. This information is made available by populating a repair table, which must first be created by the
After you run the
CHECK_OBJECT procedure, a simple query on the repair table shows the corruptions and repair directives for the object. With this information, you can assess how best to address the reported problems.
DB_VERIFY as an offline diagnostic utility when you encounter data corruption.
ANALYZE TABLE...VALIDATE STRUCTURE statement validates the structure of the analyzed object. If the database successfully validates the structure, then a message confirming its validation is returned. If the database encounters corruption in the structure of the object, then an error message is returned. In this case, drop and re-create the object.
You can enable database block checking by setting the
DB_BLOCK_CHECKING initialization parameter to
TRUE. This checks data and index blocks for internal consistency whenever they are modified.
DB_BLOCK_CHECKING is a dynamic parameter, modifiable by the
ALTER SYSTEM SET statement. Block checking is always enabled for the system tablespace.
Task 2: Evaluate the Costs and Benefits of Using DBMS_REPAIR
DBMS_REPAIR you must weigh the benefits of its use in relation to the liabilities. You should also examine other options available for addressing corrupt objects. Begin by answering the following questions:
- What is the extent of the corruption?
- What other options are available for addressing block corruptions? Consider the following:
- If the data is available from another source, then drop, re-create, and repopulate the object.
- Issue the
CREATE TABLE...AS SELECTstatement from the corrupt table to create a new one.
- Ignore the corruption by excluding corrupt rows from
- Perform media recovery.
- What logical corruptions or side effects are introduced when you use
DBMS_REPAIRto make an object usable? Can these be addressed? What is the effort required to do so?
It is possible that you do not have access to rows in blocks marked corrupt. However, a block can be marked corrupt even if there are rows that you can validly access.
It is also possible that referential integrity constraints are broken when blocks are marked corrupt. If this occurs, then disable and reenable the constraint; any inconsistencies are reported. After fixing all problems, you should be able to reenable the constraint.
Logical corruption can occur when there are triggers defined on the table. For example, if rows are reinserted, should insert triggers be fired or not? You can address these issues only if you understand triggers and their use in your installation.
If indexes and tables are not synchronized, then execute the
DUMP_ORPHAN_KEYSprocedure to obtain information from the keys that might be useful in rebuilding corrupted data. Then issue the
ALTER INDEX...REBUILD ONLINEstatement to synchronize the table with its indexes.
- If repair involves loss of data, can this data be retrieved?
Task 3: Make Objects Usable
DBMS_REPAIR makes the object usable by ignoring corruptions during table and index scans.
Corruption Repair: Using the FIX_CORRUPT_BLOCKS and SKIP_CORRUPT_BLOCKS Procedures
You can make a corrupt object usable by establishing an environment that skips corruptions that remain outside the scope of
If corruptions involve a loss of data, such as a bad row in a data block, all such blocks are marked corrupt by the
FIX_CORRUPT_BLOCKS procedure. Then you can run the
SKIP_CORRUPT_BLOCKS procedure, which skips blocks that are marked as corrupt. When the
SKIP_FLAG parameter in the procedure is set, table and index scans skip all blocks marked corrupt. This applies to both media and software corrupt blocks.
Implications when Skipping Corrupt Blocks
If an index and table are not synchronized, then a
SET TRANSACTION READ ONLY transaction can be inconsistent in situations where one query probes only the index, and a subsequent query probes both the index and the table. If the table block is marked corrupt, then the two queries return different results, thereby breaking the rules of a read-only transaction. One way to approach this isnot to skip corruptions in a
SET TRANSACTION READ ONLY transaction.
A similar issue occurs when selecting rows that are chained. A query of the same row may or may not access the corruption, producing different results.
Task 4: Repair Corruptions and Rebuild Lost Data
After making an object usable, perform the following repair activities.
Recover Data Using the DUMP_ORPHAN_KEYS Procedures
DUMP_ORPHAN_KEYS procedure reports on index entries that point to rows in corrupt data blocks. All such index entries are inserted into an orphan key table that stores the key and rowid of the corruption.
After the index entry information has been retrieved, you can rebuild the index using the
ALTER INDEX...REBUILD ONLINE statement.
Fix Segment Bitmaps Using the SEGMENT_FIX_STATUS Procedure
Use this procedure if free space in segments is being managed by using bitmaps (
SEGMENT SPACE MANAGEMENT AUTO).
This procedure recalculates the state of a bitmap entry based on the current contents of the corresponding block. Alternatively, you can specify that a bitmap entry be set to a specific value. Usually the state is recalculated correctly and there is no need to force a setting.
This section includes the following topics:
- Examples: Building a Repair Table or Orphan Key Table
- Example: Detecting Corruption
- Example: Fixing Corrupt Blocks
- Example: Finding Index Entries Pointing to Corrupt Data Blocks
- Example: Skipping Corrupt Blocks
Examples: Building a Repair Table or Orphan Key Table
ADMIN_TABLE procedure is used to create, purge, or drop a repair table or an orphan key table.
A repair table provides information about the corruptions that were found by the
CHECK_OBJECT procedure and how these will be addressed if the
FIX_CORRUPT_BLOCKS procedure is run. Further, it is used to drive the execution of the
An orphan key table is used when the
DUMP_ORPHAN_KEYS procedure is executed and it discovers index entries that point to corrupt rows. The
DUMP_ORPHAN_KEYS procedure populates the orphan key table by logging its activity and providing the index information in a usable manner.
The following example creates a repair table for the
BEGIN DBMS_REPAIR.ADMIN_TABLES ( TABLE_NAME => 'REPAIR_TABLE', TABLE_TYPE => dbms_repair.repair_table, ACTION => dbms_repair.create_action, TABLESPACE => 'USERS'); END; /
For each repair or orphan key table, a view is also created that eliminates any rows that pertain to objects that no longer exist. The name of the view corresponds to the name of the repair or orphan key table and is prefixed by
DBA_ (for exampl,
The following query describes the repair table that was created for the
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
This example illustrates the creation of an orphan key table for the
BEGIN DBMS_REPAIR.ADMIN_TABLES ( TABLE_NAME => 'ORPHAN_KEY_TABLE', TABLE_TYPE => dbms_repair.orphan_table, ACTION => dbms_repair.create_action, TABLESPACE => 'USERS'); END; /
The orphan key table is described in the following query:
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
Example: Detecting Corruption
CHECK_OBJECT procedure checks the specified object, and populates the repair table with information about corruptions and repair directives. You can optionally specify a range, partition name, or subpartition name when you want to check a portion of an object.
Validation consists of checking all blocks in the object that have not previously been marked corrupt. For each block, the transaction and data layer portions are checked for self consistency. During
CHECK_OBJECT, if a block is encountered that has a corrupt buffer cache header, then that block is skipped.
The following is an example of executing the
CHECK_OBJECT procedure for the
SET SERVEROUTPUT ON DECLARE num_corrupt INT; BEGIN num_corrupt := 0; DBMS_REPAIR.CHECK_OBJECT ( SCHEMA_NAME => 'SCOTT', OBJECT_NAME => 'DEPT', REPAIR_TABLE_NAME => 'REPAIR_TABLE', CORRUPT_COUNT => num_corrupt); DBMS_OUTPUT.PUT_LINE('number corrupt: ' || TO_CHAR (num_corrupt)); END; /
SQL*Plus outputs the following line, indicating one corruption:
number corrupt: 1
Querying the repair table produces information describing the corruption and suggesting a repair action.
SELECT OBJECT_NAME, BLOCK_ID, CORRUPT_TYPE, MARKED_CORRUPT, CORRUPT_DESCRIPTION, REPAIR_DESCRIPTION FROM REPAIR_TABLE; OBJECT_NAME BLOCK_ID CORRUPT_TYPE MARKED_COR ------------------------------ ---------- ------------ ---------- CORRUPT_DESCRIPTION ------------------------------------------------------------------------------ REPAIR_DESCRIPTION ------------------------------------------------------------------------------ DEPT 3 1 FALSE kdbchk: row locked by non-existent transaction table=0 slot=0 lockid=32 ktbbhitc=1 mark block software corrupt
The corrupted block has not yet been marked corrupt, so this is the time to extract any meaningful data. After the block is marked corrupt, the entire block must be skipped.
Example: Fixing Corrupt Blocks
FIX_CORRUPT_BLOCKS procedure to fix the corrupt blocks in specified objects based on information in the repair table that was generated by the
CHECK_OBJECT procedure. Before changing a block, the block is checked to ensure that the block is still corrupt. Corrupt blocks are repaired by marking the block software corrupt. When a repair is performed, the associated row in the repair table is updated with a timestamp.
This example fixes the corrupt block in table
scott.dept that was reported by the
SET SERVEROUTPUT ON DECLARE num_fix INT; BEGIN 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=> num_fix); DBMS_OUTPUT.PUT_LINE('num fix: ' || TO_CHAR(num_fix)); END; /
SQL*Plus outputs the following line:
num fix: 1
The following query confirms that the repair was done.
SELECT OBJECT_NAME, BLOCK_ID, MARKED_CORRUPT FROM REPAIR_TABLE; OBJECT_NAME BLOCK_ID MARKED_COR ------------------------------ ---------- ---------- DEPT 3 TRUE
Example: Finding Index Entries Pointing to Corrupt Data Blocks
DUMP_ORPHAN_KEYS procedure reports on index entries that point to rows in corrupt data blocks. For each index entry, a row is inserted into the specified orphan key table. The orphan key table must have been previously created.
This information can be useful for rebuilding lost rows in the table and for diagnostic purposes.
This should be run for every index associated with a table identified in the repair table.
In this example,
pk_dept is an index on the
scott.dept table. It is scanned to determine if there are any index entries pointing to rows in the corrupt data block.
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; /
The following output indicates that there are three orphan keys:
orphan key count: 3
Index entries in the orphan key table implies that the index should be rebuilt. This guarantees that a table probe and an index probe return the same result set.
Example: Skipping Corrupt Blocks
SKIP_CORRUPT_BLOCKS procedure enables or disables the skipping of corrupt blocks during index and table scans of the specified object. When the object is a table, skipping applies to the table and its indexes. When the object is a cluster, it applies to all of the tables in the cluster, and their respective indexes.
The following example enables the skipping of software corrupt blocks for the
BEGIN DBMS_REPAIR.SKIP_CORRUPT_BLOCKS ( SCHEMA_NAME => 'SCOTT', OBJECT_NAME => 'DEPT', OBJECT_TYPE => dbms_repair.table_object, FLAGS => dbms_repair.skip_flag); END; /
scott‘s tables using the
DBA_TABLES view shows that
SKIP_CORRUPT is enabled for table
SELECT OWNER, TABLE_NAME, SKIP_CORRUPT FROM DBA_TABLES WHERE OWNER = 'SCOTT'; OWNER TABLE_NAME SKIP_COR ------------------------------ ------------------------------ -------- SCOTT ACCOUNT DISABLED SCOTT BONUS DISABLED SCOTT DEPT ENABLED SCOTT DOCINDEX DISABLED SCOTT EMP DISABLED SCOTT RECEIPT DISABLED SCOTT SALGRADE DISABLED SCOTT SCOTT_EMP DISABLED SCOTT SYS_IOT_OVER_12255 DISABLED SCOTT WORK_AREA DISABLED 10 rows selected.