Saturday, December 22, 2007

Handling Block Corruptions in Oracle7 / 8 / 8i / 9i / 10g

Contents
Introduction
Overview of Steps to handle a Corruption
Corruption due to NOLOGGING or UNRECOVERABLE
(1) Determine the Extent of the Corruption Problem
(2) Replace or Move Away from Suspect Hardware
(3) Which Objects are Affected ?
Options for various Segment Types:
CACHE
CLUSTER
INDEX PARTITION INDEX
LOBINDEX LOBSEGMENT
ROLLBACK
TABLE PARTITION TABLE
TEMPORARY IOT
TYPE2 UNDO
Other Segment Types
No Segment
(4) Choosing a Recovery Option
(4A) Complete Recovery
Block Level Recovery ,
Datafile Recovery ,
Database Recovery ,
After Complete Recovery
(4B) Recreating Indexes
(4C) Salvaging Data from Tables
Methods of extracting data from a corrupt table AROUND a corrupt block
Methods of extracting data from a table with a corrupt LOBSEGMENT block
Extracting data from the corrupt block itself
(4D) Leaving the Corruption in Place
Warnings when Leaving a Corruption in Place
(4E) Last Options
Document History

All SQL statements here are for use in SQL*Plus (in 8.1 or higher)
or Server Manager (Oracle7 / 8.0) when connected as a SYSDBA user.
(Eg: "connect / as sysdba" or "connect internal")


Introduction
This article discusses how to handle one or more block corruptions
on an Oracle datafile and describes the main actions to take to deal
with them. Please read the complete article before taking any action.

This note does not cover memory corruption issues (typically
ORA-600 [17xxx] type errors).

Note: If the problem is an ORA-1578 on STARTUP then please
contact your local support center for advice referencing
Note:106638.1 - this note is not visible to customers
but the relevant steps from it can be supplied by an experienced
support analyst.

You may be referred to this article from many places for many forms of
error - it is important that you have the following information for each
corrupt block:
An absolute FILE NUMBER of the file containing the corrupt block.
Referred to as "&AFN" in this article.

The file name of the file containing the corrupt block.
Referred to as "&FILENAME" in this article.
( If you know the FILE NUMBER but not its name then V$DATAFILE
can be used to get the file name:
SELECT name FROM v$datafile WHERE file#=&AFN;
If the file number does not appear in V$DATAFILE in Oracle8i
AND &AFN is greater than the DB_FILES parameter
value then it is probably a TEMPFILE. In this case the filename
can be found using:
SELECT name FROM v$tempfile
WHERE file#=(&AFN - &DB_FILES_value);
)
The BLOCK NUMBER of the corrupt block in that file.
Referred to as "&BL" in this article.
The tablespace number and name containing the affected block.
Referred to as "&TSN" (tablespace number) and
"&TABLESPACE_NAME" in this article.
If you do not know these then you can find them using:

SELECT ts# "TSN" FROM v$datafile WHERE file#=&AFN;

SELECT tablespace_name FROM dba_data_files WHERE file_id=&AFN;
The block size of the tablespace where the corruption lies.
Referred to as "&TS_BLOCK_SIZE" in this article.

For Oracle 9i+, run the following query to determine the appropriate
block size:

SELECT block_size FROM dba_tablespaces
WHERE tablespace_name =
(SELECT tablespace_name FROM dba_data_files WHERE file_id=&AFN);

For Oracle 7, 8.0 and 8.1:
Every tablespace in the database has the same block size.
For these versions, issue "SHOW PARAMETER DB_BLOCK_SIZE" and
use this value as your &TS_BLOCK_SIZE.

Eg: For the ORA-1578 error:
ORA-01578: ORACLE data block corrupted (file # 7, block # 12698)
ORA-01110: data file 22: '/oracle1/oradata/V816/oradata/V816/users01.dbf'
then:
&AFN is "22" (from the ORA-1110 portion of the error)
&RFN is "7" (from the "file #" in the ORA-1578)
&BL is "12698" (from the "block #" in the ORA-1578)
&FILENAME is '/oracle1/oradata/V816/oradata/V816/users01.dbf'
&TSN etc.. should be determined from the above SQL

For other errors (ORA-600 , ORA-1498 etc...) the above values should
either be given to you by Oracle Support, or be given to you from the
article which covers the relevant error.



Overview of Steps to handle a Corruption
There are many possible causes of a block corruption including:

- Bad IO hardware / firmware
- OS problems
- Oracle problems
- Recovering through "UNRECOVERABLE" or "NOLOGGING" database actions
(in which case ORA-1578 is expected behaviour - see below)

The point in time when an Oracle error is raised may be much later than
when any corruption initially occurred.

As the root cause is not usually known at the time the corruption is
encountered, and as in most cases the key requirement is to get up
and running again, then the steps used tackle corruption problems in
this article are:

1) Determine the extent of the corruption problems
and also determine if the problems are permanent or transient.

If the problem is widespread or the errors move about
then focus on identifying the cause first (check hardware
etc..). This is important as there is no point recovering
a system if the underlying hardware is faulty.

2) Replace or move away from any faulty or suspect hardware.

3) Determine which database objects are affected.

4) Choose the most appropriate database recovery / data salvage
option.

For all steps above it is sensible to collect evidence and
document exactly what actions are being taken. The 'Evidence>>'
tags in this article list the information which should be collected
to assist with identifying the root cause of the problem.


Corruption due to NOLOGGING or UNRECOVERABLE

If a NOLOGGING (or UNRECOVERABLE) operation is performed on an
object and the datafile containing that object is subsequently
recovered then the data blocks affected by the NOLOGGING operation
are marked as corrupt and will signal an ORA-1578 error when
accessed. In Oracle8i an ORA-26040 is also signalled
("ORA-26040: Data block was loaded using the NOLOGGING option" )
which makes the cause fairly obvious, but earlier releases have no
additional error message. If a block is corrupt due to recovery
through a NOLOGGING operation then you can use this article from
Section 3 "Which Objects are Affected ?" onwards but note that:

(a) Recovery cannot retrieve the NOLOGGING data

(b) No data is salvagable from inside the block


(1) Determine the Extent of the Corruption Problem
Whenever a corruption error occurs note down the FULL error message/s
and look in the instance's alert log and trace files for any associated
errors. It is important to do this first to assess whether this is
a single block corruption, an error due to an UNRECOVERABLE operation
or a more severe issue.

It is a good idea to scan affected files (and any important files)
with DBVERIFY to check for other corruptions in order to determine
the extent of the problem.
For details of using DBVERIFY see Note 35512.1

Once you have determined a list of corrupt file/block combinations
then the steps below can be used to help determine what action
can be taken.

Evidence>>
- Record the original error in full, along with details of
the application which encountered the error.
- Save an extract from the alert log from a few hours before
the FIRST recorded problem up to the current point in time.
- Save any tracefiles mentioned in the alert log.
- Record any recent OS problems you have encountered.
- Note if you are using any special features - Eg: ASYNC IO,
fast write disk options etc..
- Record your current BACKUP position (Dates, Type etc...)
- Note if your database is in ARCHIVELOG mode or not
Eg: Issue "ARCHIVE LOG LIST" in SQL*Plus (or Server Manager)


(2) Replace or Move Away from Suspect Hardware
The vast majority of corruption problems are caused by faulty hardware.
If there is a hardware fault or a suspect component then it is sensible
to either repair the problem, or make disk space available on a
separate disk sub-system prior to proceeding with a recovery option.

You can move datafiles about using the following steps:

1. Make sure the file to be relocated is either OFFLINE or
the instance is in the MOUNT state (not open)

2. Physically restore (or copy) the datafile to its new location
eg: /newlocation/myfile.dbf

3. Tell Oracle the new location of the file.
eg: ALTER DATABASE RENAME FILE '/oldlocation/myfile.dbf'
TO '/newlocation/myfile.dbf';
(Note that you cannot RENAME a TEMPFILE - TEMPFILEs should
be dropped and recreated at the new location)

4. Online the relevant file / tablespace (if database is open)


IMPORTANT: If there are multiple errors (which are NOT due to NOLOGGING)
OR You have OS level errors against the affected file
OR The errors are transient and keep moving about
then there is little point proceeding until the underlying problem
has been addressed or space is available on alternative disks.
Get your hardware vendor to check the system over and contact
Oracle Support with details of all errors.
Please note: Whilst a failed hardware check is a good indication
that there is a hardware issue, a successful hardware check should not
be taken as proof that there is no hardware related issue - it is very
common for hardware tests to report success when there really is some
underlying fault.


If using any special IO options such as direct IO , async IO or
similar it may be worth disabling them in order to eliminate such options
as a potential source of problems.



(3) Which Objects are Affected ?
It is best to determine which objects are affected BEFORE making any
decisions about how to recover - this is because the corruption/s may be
on object/s which can easily be re-created.
Eg: For a corruption on a 5 row lookup table it may be far quicker to
drop and recreate the table than to perform a recovery.

For each corruption collect the information in the following table.
The steps to do this are explained below.
Information to Record for each Corruption Original
Error Absolute
File#
&AFN Relative
File#
&RFN Block#

&BL Tablespace Segment
Type Segment
Owner.Name Related
Objects Recovery
Options





















The notes below will help you fill in this table for each corruption.

"Original Error"
This is the error as initially reported.
Eg: ORA-1578 / ORA-1110 , ORA-600 with all arguments etc..

"Absolute File#", "Relative File#" and "Block#"
The File# and Block# should have been given to you either by the
error, by Oracle Support, or by the steps in an error article which
directed you to this article.

In Oracle8/8i/9i/10g:
The absolute and relative file numbers are often the
same but can differ (especially if the database has
been migrated from Oracle7). It is important to get
the correct numbers for &AFN and &RFN
or you may end up salvaging the wrong object !!

An ORA-1578 reports the RELATIVE file number, with the
ABSOLUTE file number given in the accompanying ORA-1110
error. For ORA-600 errors you should be told an absolute
file number.

The following query will show the absolute and relative
file numbers for datafiles in the database:

SELECT tablespace_name, file_id "AFN", relative_fno "RFN"
FROM dba_data_files;

In Oracle8i/9i/10g:
In addition to the notes above about Oracle8, Oracle8i onwards
can have TEMPFILES. The following query will show the
absolute and relative file numbers for tempfiles in the
database:

SELECT tablespace_name, file_id+value "AFN", relative_fno "RFN"
FROM dba_temp_files, v$parameter
WHERE name='db_files';

In Oracle7: Use the same file number for both the "Absolute File#"
and the "Relative File#"



"Segment Type", "Owner", "Name" and "Tablespace"
The following query will tell you the object TYPE , OWNER and NAME of
a segment given the absolute file number "&AFN" and block number "&BL" of the
corrupt block - the database must be open in order to use this query:

SELECT tablespace_name, segment_type, owner, segment_name
FROM dba_extents
WHERE file_id = &AFN
and &BL between block_id AND block_id + blocks - 1
;

If the block is in a TEMPFILE the above query will return no data.
For TEMPFILES the "Segment Type" will be "TEMPORARY".


"Related Objects" and Possible "Recovery Options" by SEGMENT_TYPE:
The related objects and recovery options which can be used depend on the
SEGMENT_TYPE. The additional queries and possible recovery options are
listed below for each of the most common segment types.

CACHE
CLUSTER
INDEX PARTITION INDEX
LOBINDEX LOBSEGMENT
ROLLBACK
TABLE PARTITION TABLE
TEMPORARY
TYPE2 UNDO
Some other Segment Type
"no rows" from the query

CACHE
- If the segment type is CACHE recheck you have entered the SQL
and parameters correctly.
If you get the same result contact Oracle support with all
information you have.

Options:
The database is likely to require recovery.

{Continue} {Back to Segment List}

CLUSTER
- If the segment is a CLUSTER determine which tables it contains.
Eg:
SELECT owner, table_name
FROM dba_tables
WHERE owner='&OWNER'
AND cluster_name='&SEGMENT_NAME'
;

Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary clusters possible options include:
Recovery
OR Salvage data from all tables in the cluster
THEN Recreate the cluster and all its tables

As the cluster may contain a number of tables, it is best to
collect information for each table in the cluster before making a
decision.

{Collect TABLE information} {Back to Segment List}

INDEX PARTITION
- If the segment is an INDEX PARTITION note the NAME and OWNER
and then determine which partition is affected thus:

SELECT partition_name
FROM dba_extents
WHERE file_id = &AFN
AND &BL BETWEEN block_id AND block_id + blocks - 1
;

then continue below as if the segment was an INDEX segment.

Options:
Index partitions can be rebuilt using:
ALTER INDEX xxx REBUILD PARTITION ppp;
(take care with the REBUILD option as described in
"Recreating Indexes" below)


INDEX
- If the segment is an INDEX then if the OWNER is "SYS" contact
Oracle support with all details.

For a non-dictionary INDEX or INDEX PARTITIONs find out which table
the INDEX is on:
Eg:
SELECT table_owner, table_name
FROM dba_indexes
WHERE owner='&OWNER'
AND index_name='&SEGMENT_NAME'
;

and determine if the index supports a CONSTRAINT:

Eg: SELECT owner, constraint_name, constraint_type, table_name
FROM dba_constraints
WHERE owner='&TABLE_OWNER'
AND constraint_name='&INDEX_NAME'
;

Possible values for CONSTRAINT_TYPE are:

P The index supports a primary key constraint.
U The index supports a unique constraint.


If the INDEX supports a PRIMARY KEY constraint (type "P") then
check if the primary key is referenced by any foreign key constraints:
Eg:
SELECT owner, constraint_name, constraint_type, table_name
FROM dba_constraints
WHERE r_owner='&TABLE_OWNER'
AND r_constraint_name='&INDEX_NAME'
;

Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary indexes possible options include:
Recovery
OR Recreate the index (with any associated constraint
disables/enables)
(take care with the REBUILD option as described in
"Recreating Indexes" below)

{Continue} {Back to Segment List}

ROLLBACK
- If the segment is a ROLLBACK segment contact Oracle support as
rollback segment corruptions require special handling.

Options:
The database is likely to require recovery.
{Continue} {Back to Segment List}

TYPE2 UNDO
- TYPE2 UNDO is a system managed undo segment which is a special
form of rollback segment. Corruptions in these segments require
special handling.

Options:
The database is likely to require recovery.
{Continue} {Back to Segment List}

TABLE PARTITION
- If the segment is a TABLE PARTITION note the NAME and OWNER
and then determine which partition is affected thus:

SELECT partition_name
FROM dba_extents
WHERE file_id = &AFN
AND &BL BETWEEN block_id AND block_id + blocks - 1
;

then continue below as if the segment was a TABLE segment.

Options:
If all corruptions are in the same partition then one option
at this point is to EXCHANGE the corrupt partition with an
empty TABLE - this can allow the application to continue (without
access to the data in the corrupt partition) whilst any good
data can then be extracted from the table.

For other options see the TABLE options below.


TABLE
- If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For a non-dictionary TABLE or TABLE PARTITIONs find out which
INDEXES exist on the TABLE:
Eg:
SELECT owner, index_name, index_type
FROM dba_indexes
WHERE table_owner='&OWNER'
AND table_name='&SEGMENT_NAME'
;

and determine if there is any PRIMARY key on the table:

Eg: SELECT owner, constraint_name, constraint_type, table_name
FROM dba_constraints
WHERE owner='&OWNER'
AND table_name='&SEGMENT_NAME'
AND constraint_type='P'
;

If there is a primary key then check if this is referenced by any
foreign key constraints:
Eg:
SELECT owner, constraint_name, constraint_type, table_name
FROM dba_constraints
WHERE r_owner='&OWNER'
AND r_constraint_name='&CONSTRAINT_NAME'
;

Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary tables possible options include:
Recovery
OR Salvage data from the table (or partition)
THEN Recreate the table (or partition)
OR Leave the corruption in place
(eg: Use DBMS_REPAIR to mark the problem blocks to be skipped)

{Continue} {Back to Segment List}

IOT (Index Organized Table)
The corruption in IOT table should be handled in the same way as in a heap or partitioned table.
The only exception is if the PK is corrupted.
PK of an IOT table is the table itself and can't be dropped and recreated.

Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary tables possible options include:
Recovery
OR Salvage data from the table (or partition)
THEN Recreate the table (or partition)
OR Leave the corruption in place
(eg: Use DBMS_REPAIR to mark the problem blocks to be skipped)

{Continue} {Back to Segment List}

LOBINDEX
- Find out which table the LOB belongs to:

SELECT table_name, column_name
FROM dba_lobs
WHERE owner='&OWNER'
AND index_name='&SEGMENT_NAME';

- If the table is owned by "SYS" then contact Oracle support with all
details. The database is likely to require recovery.

- It is not possible to rebuild LOB indexes and so you have to
treat the problem as a corruption on the LOB column of the
affected table.

Get index and constraint information for the table which has
the corrupt LOB index using the SQL in the TABLE
section, then return here.

Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary tables possible options include:
Recovery
OR Salvage data from the table (and its LOB column/s)
THEN Recreate the table
It is not generally sensible just to leave the corruption in
place unless the table is unlikely to have any further DML on
the problem column.


{Continue} {Back to Segment List}


LOBSEGMENT

- Find out which table the LOB belongs to:
Eg:
SELECT table_name, column_name
FROM dba_lobs
WHERE owner='&OWNER'
AND segment_name='&SEGMENT_NAME';

- If the table is owned by "SYS" then contact Oracle support with all
details. The database is likely to require recovery.

- For non-dictionary tables ...

Get index and constraint information for the table which has
the corrupt LOB data using the SQL in the TABLE
section, then return here to find details of the exact rows
affected.

Finding the exact row which references the corrupt LOB block
can be a challenge as the errors reported do not show any
detail about which table row owns the lob entry which is corrupt.

Typically one can refer to application logs or any SQL_TRACE
or 10046 trace of a session hitting the error (if available) or
see if having event "1578 trace name errorstack level 3"
set in the session helps identify the current SQL/binds/row.
eg:
ALTER SYSTEM SET EVENTS '1578 trace name errorstack level 3';

Then wait for the error to be hit by the application
and find the trace file.

If there are no clues then you can construct a PLSQL block
to scan the problem table row by row extracting the LOB
column data which loops until it hits an error. Such a technique
may take a while but it should be possible to get a primary key
or rowid of any row which references a corrupt LOB block.

eg:
set serverout on
exec dbms_output.enable(100000);
declare
error_1578 exception;
pragma exception_init(error_1578,-1578);
n number;
cnt number:=0;
badcnt number:=0;
begin
for cursor_lob in
(select rowid r, &LOB_COLUMN_NAME L from &OWNER..&TABLE_NAME)
loop
begin
n:=dbms_lob.instr(cursor_lob.L,hextoraw('AA25889911'),1,999999) ;
exception
when error_1578 then
dbms_output.put_line('Got ORA-1578 reading LOB at '||cursor_lob.R);
badcnt:=badcnt+1;
end;
cnt:=cnt+1;
end loop;
dbms_output.put_line('Scanned '||cnt||' rows - saw '||badcnt||' errors');
end;
/


It is possible to have a corrupt LOB block which is only
present as an old version (for consistent read) and which has
not yet been re-used in which case all table rows will be
accessible but it may not be possible to insert / update
the LOB columns once that block is reclaimed for reuse.


Options:
If the OWNER is "SYS" then contact Oracle support with all details.
The database is likely to require recovery.

For non dictionary tables possible options include:
Recovery
OR Salvage data from the table (and its LOB column/s)
THEN Recreate the table
OR Leave the corruption in place
(It is not possible to use DBMS_REPAIR on LOB segments)


{Continue} {Back to Segment List}

TEMPORARY
- If the segment type is TEMPORARY then the corruption does not
affect a permanent object. Check if the tablespace where the
problem occurred is being used as a TEMPORARY tablespace thus:

SELECT count(*) FROM dba_users
WHERE temporary_tablespace='&TABLESPACE_NAME'
;

Options:
If this is a TEMPORARY_TABLESPACE then it may be possible
to create a NEW temporary tablespace and switch all users
to that tablespace then DROP the problem tablespace.

If this is not a temporary tablespace then the block should
not be read again and should get re-formatted next time the
block is used - the error should not repeat PROVIDED any
underlying cause has been cured.

No restore is normally required, although if the disk is
suspect and the tablespace contains useful data then a
database recovery of the affected file/s may be wise.


{Continue} {Back to Segment List}

Some other SEGMENT_TYPE
- If the segment type returned is not covered above then contact
Oracle support for advice with all information collected so far.

{Continue} {Back to Segment List}

"no rows returned"
- If there appears to be no extent containing the corrupt block
then first double check the figures used in the query. If you
are sure the file and block are correct and do not appear as
belonging to an object in DBA_EXTENTS then:

- Double check if the file involved is a TEMPFILE.
Note that TEMPFILE file numbers depend on the init.ora
parameter DB_FILES so any changes to this parameter
change the absolute file number reported in errors.

- DBA_EXTENTS does not include blocks which are used
for local space management in locally managed tablespaces.

- If the database you are now querying is from a different
point in time to the datafile with the error then the
problem object may have been dropped and so queries against
DBA_EXTENTS may show no rows.

- If the error you are investigating was reported by DBVERIFY
then DBV checks all blocks regardless of whether they
belong to an object or not. This it is possible for a
corrupt block to exist in the datafile but in a block
not in use by any object.

Options:
An error on an UNUSED Oracle block can be ignored as Oracle will
create a new block image should the block need to be used so any
existing problem on the block will never get read.

If you suspect that the block may be a space management
block then you can use DBMS_SPACE_ADMIN to help check
this by running:

exec DBMS_SPACE_ADMIN.TABLESPACE_VERIFY('&TABLESPACE_NAME');

This should write inconsistencies to the trace file but
if it encounters a fatally corrupt block it will report an
error like:
ORA-03216: Tablespace/Segment Verification cannot proceed

An error on a bitmap space management block can often be corrected
by running:

exec DBMS_SPACE_ADMIN.TABLESPACE_REBUILD_BITMAPS('&TABLESPACE_NAME');

{Continue} {Back to Segment List}

Evidence>>
- For each corrupt block it is also a good idea to collect
the following physical evidence if there is a need to try
and identify the actual cause of the corruption:

i) An operating system HEX dump of the bad block and the block
either side of it.
On UNIX:
dd if=&FILENAME bs=&TS_BLOCK_SIZE skip=&BL-1 count=3 of=BL.dd
^^^^^^^^ ^^^^^^^^^^^^^^ ^^^
Eg: For BL=1224:
dd if=ts11.dbf bs=4k skip=1223 count=3 of=1223_1225.dd

On VMS:
DUMP/BLOCKS=(start:XXXX,end:YYYY)/out=dump.out &FILENAME

Where XXXX=Operating system block number (in 512 byte chunks)
To calculate this multiply the block number reported by
'&TS_BLOCK_SIZE/512'.

ii) If you are in ARCHIVELOG mode make a safe copy of the archived
log files around the time of the error, and preferably for a few
hours before the error was reported. Also secure any backup/s
of the problem datafile from before the errors as the before
image PLUS redo can help point towards a cause.
(DBV can often be used to check if the problem exists in a
backup copy of a file). The ideal scenario is to have a
datafile backup image which does not have any corruption
and all the redo from that point in time up to and just past
the time when the corruption is first reported.

iii) Obtain an Oracle dump of the problem block/s:

ALTER SYSTEM DUMP DATAFILE '&FILENAME'
BLOCK &BL
;

(The output will go to a tracefile in the USER_DUMP_DEST).

{Continue} {Back to Segment List}

(4) Choosing a Recovery Option
The best recovery option now depends on the objects affected. The notes
in Section (3) above should have highlighted the main options available
for each affected object. The actual recovery method chosen may include
a mix or one or more methods thus:

Is any Recovery Required ?
If the error is in a TEMPORARY tablespace, or is in a block
which is no longer part of any database object then no action
is required, although it may be wise to relocate the problem
tablespace to a different storage device.
See Warnings.

Is Complete Recovery an option ?

In order for complete recovery to be an option the following
must be true:
- The database is in ARCHIVELOG mode
(The "ARCHIVE LOG LIST" command shows Archivelog Mode)

- You have a good backup of affected files. Note that in some cases,
the corruption may have been present, but undetected, for a long
period of time. If the most recent datafile backup still contains
the corruption, you can try an earlier backup as long as you have all
the necessary ARCHIVELOGS.

(You can often use the DBV START= / END= options to check
if specific block/s in a restored copy of a backup
file are corrupt)

- All ARCHIVELOGS are available from the time of the backup
to the current point in time

- The current online log/s are available and intact

- The errors are NOT due to recovery through a NOLOGGING operation

When the above criteria are satisfied then complete recovery
is usually the preferred option
*BUT NOTE*
(a) If the rollback of a transaction has seen a corrupt block on an
object other than the rollback segment itself then UNDO may
have been discarded. In this case you may need to rebuild
indexes / check data integrity AFTER the recovery completes.
(b) If the files to be recovered contain data from NOLOGGING
operations performed since the last backup then those blocks
will be marked corrupt if datafile or database recovery is
used. In some cases this can put you in a worse scenario than
the current position.

If database recovery has already been performed and the corruption
is still there then either all of your backups contain the corruption,
the underlying fault is still present or the problem is replaying
through redo. In these cases you will need to choose some other
recovery option.

See "(4A) Complete Recovery" for complete recovery steps.

Can the object be Dropped or Re-created without needing
to extract any data from the object itself ?
It may be possible to lose the object, or to recreate it from
a script / recent export. Once an object is dropped then blocks
in that object are marked as "free" and will be re-formatted when
the block gets allocated to a new object. It is advisable to RENAME
rather than DROP a table unless you are absolutely sure that you
do not need any data in it.

In the case of a table partition then only the affected partition
needs to be dropped. eg: ALTER TABLE ... DROP PARTITION ...
If the corruption affects the partition segment header, or the
file containing the partition header is offline, then DROP
PARTITION may fail. In this case it may still be possible to
drop the partition by first exchanging it with a table of the
same definition.
eg: ALTER TABLE .. EXCHANGE PARTITION .. WITH TABLE ..;

The most common object which can be re-created is an index.
Always address TABLE corruptions before INDEX problems on a table.
See "(4B) Recreating Indexes" for more details.

Is it required to salvage data before recreating the object ?
If the problem is on a critical application table which is
regularly updated then it may be required to salvage as much
data from the table as possible, then recreate the table.

See "(4C) Salvaging Data from Tables" for more details.

Is it acceptable to leave the corruption in place for the moment?
In some cases the best immediate option may be to leave the
corruption in place and isolate it from application access.

See "(4D) Leaving the Corruption In Place" for more details.

Last Options
Are any of the following possible ?
Recovery to an old point-in-time (via point in time recovery)
of either the database or tablespace point in time recovery
OR Restore of a COLD backup from before the corruption
OR Use of an existing export file
See "(4E) Last Options" for more details.


(4A) Complete Recovery
If the database is in ARCHIVELOG mode and you have a good backup
of the affected files then recovery is usually the preferred option.
This is not GUARANTEED to clear a problem, but is effective for the
majority of corruption issues. If recovery re-introduces the problem
then return to the list of options above and choose another method.

If you are using Oracle9i (or higher) then it may be possible
to perform block level recovery using the RMAN BLOCKRECOVER command.
If using an earlier Oracle release then you can either perform datafile
recovery (which can be done while the rest of the database is still up
and running), or database recovery (which requires the database to be
taken down) .



Block Level Recovery
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As of Oracle9i RMAN allows individual blocks to be recovered whilst the
rest of the database (including other blocks in the datafile) are available
for normal access. Note that block level recovery can only be used
to recover a block fully to the current point in time.

It is not necessary to be using RMAN for backups to be able to use this
option for recovery of individual blocks.
eg:
Consider that you have an ORA-1578 on file #6 block #30 which is likely
due to a media corruption problem and there is a good cold backup
image of that file which has been restored to '.../RESTORE/filename.dbf'.
Provided all archivelogs exist (in the default location) then you can use
RMAN to perform a block level recovery using a command sequence like:

rman nocatalog
connect target
catalog datafilecopy '.../RESTORE/filename.dbf';
run {blockrecover datafile 6 block 30;}

This will use the registered datafile backup image and any required
archivelogs to perform block recovery of just the one problem block
to current point in time.

Please see the documentation for full details of the RMAN BLOCKRECOVER
command and limitations.

Datafile Recovery
~~~~~~~~~~~~~~~~~~
Datafile recovery of a file involves the following steps. If there are
several files repeat the steps for each file or see "Database Recovery"
below. These steps can be used if the database is either OPEN or MOUNTED.

OFFLINE the affected data file
eg: ALTER DATABASE DATAFILE 'name_of_file' OFFLINE;

Copy it to a safe location (in case the backup is bad)

Restore the latest backup of the file onto a GOOD disk

Check the restored file for obvious corruptions with DBVERIFY
For details of using DBVERIFY see Note 35512.1

Assuming the restored file is OK, then RENAME the datafile to the
NEW location (if different from the old location)
eg: ALTER DATABASE RENAME FILE 'old_name' TO 'new_name';

Recover the datafile
eg: RECOVER DATAFILE 'name_of_file';

Online the file/s
eg: ALTER DATABASE DATAFILE 'name_of_file' ONLINE;

{Continue}

Database Recovery
~~~~~~~~~~~~~~~~~
Database recovery generally involves the following steps:

Shutdown (Immediate or Abort)

Copy the current copy of all files to be recovered to a safe location

Restore the backup files to a GOOD disk location
DO NOT RESTORE THE CONTROL FILES or ONLINE REDO LOG FILES

Check restored files with DBVERIFY
For details of using DBVERIFY see Note 35512.1

Startup MOUNT

Rename any relocated files
eg: ALTER DATABASE RENAME FILE 'old_name' TO 'new_name';

Ensure all required files are online
eg: ALTER DATABASE DATAFILE 'name_of_file' ONLINE;

Recover the database
eg: RECOVER DATABASE

Open the database
eg: ALTER DATABASE OPEN;


After a Complete Recovery
~~~~~~~~~~~~~~~~~~~~~~~~~~
Once a complete recovery has been performed it is advisable to check the
database before allowing it to be used:

- Run "ANALYZE VALIDATE STRUCTURE CASCADE"
against each problem object to check for table/index mis-matches.
If there has been any UNDO discarded this may show a mismatch
requiring indexes to be re-created.

- Check the logical integrity of data in the table at the application level.



(4B) Recreating Indexes
If the corrupt object is a user INDEX you can simply drop and
re-create it PROVIDED the underlying table is not also corrupt.
If the underlying table is also corrupt it is advisable to sort out
the TABLE before recreating any indexes.

If the information collected shows that the index has dependent FOREIGN
KEY constraints then you will need to do something like this:

- ALTER TABLE DISABLE CONSTRAINT ;
for each foreign key

- Rebuild the primary key using
ALTER TABLE DISABLE CONSTRAINT ;
DROP INDEX ;
CREATE INDEX .. with appropriate storage clause
ALTER TABLE
ENABLE CONSTRAINT ;

- Enable the foreign key constraints
ALTER TABLE ENABLE CONSTRAINT ;

For an index partition you can:
ALTER INDEX ... REBUILD PARTITION ...;

Notes:
(1) It is important not to REBUILD a non-partitioned corrupt index
using an "ALTER INDEX .. REBUILD" command as this will usually
try to build the new index from the existing index segment,
which contains a corrupt block.
"ALTER INDEX ... REBUILD ONLINE" and "ALTER INDEX ... REBUILD
PARTITION ..." do not build the new index from the old index
segment and so can be used.

(2) Create INDEX can use the data from an existing index
if the new index is a sub-set of the columns in the
existing index. Hence if you have 2 corrupt indexes drop
them BOTH before re-creating them.

(3) Be sure to use the correct storage details when recreating indexes.


(4C) Salvaging Data from Tables
If the corrupt object is a TABLE or CLUSTER or LOBSEGMENT then it
must be understood that the data within the corrupt block is lost.
Some of the data may be salvageable from a HEX dump of the block, or
from columns covered by indexes.

Important:
As it may be required to salvage data in the corrupt
block from the indexes it is a good idea NOT to drop
any existing index until any required data has been
extracted.

There are many ways to get data out of a table which contains a
corrupt block. Choose the most appropriate method as detailed
below. The aim of these methods is to extract as much data as
possible from the table blocks which can be accessed. It is
usually a good idea to RENAME the corrupt table so that the new
object can be created with the correct name.
Eg: RENAME TO ;

Methods of extracting data from a corrupt table AROUND a corrupt block

(1) From Oracle 7.2 onwards, including Oracle 8.0, 8.1, and 9i, it is
possible to SKIP over corrupt blocks in a table.
This is by far the simplest option to extract table data and is
discussed in:

Extracting data using DBMS_REPAIR.SKIP_CORRUPT_BLOCKS or
Event 10231 Note 33405.1

Note that this method can only be used if the block "wrapper" is
marked corrupt. Eg: If the block reports ORA-1578.

If the problem is an ORA-600 or other error which does not report
and ORA-1578 error then it is often possible to use DBMS_REPAIR
to mark the problem blocks in a table as "soft corrupt" such
that they will then signal ORA-1578 when accessed which then
allows you to use DBMS_REPAIR.SKIP_CORRUPT_BLOCKS.

Note: Any blocks which are marked corrupt by the "FIX_CORRUPT_BLOCKS"
procedure will also be marked corrupt following any restore /
recover operation through the time of the FIX_CORRUPT_BLOCKS.

Full details of using DBMS_REPAIR for this can be found in the
documentation but in summary the steps are:

- Use DBMS_REPAIR.ADMIN_TABLES to create the admin tables
- Use DBMS_REPAIR.CHECK_OBJECT to find problem blocks
- Get any good data out of problem blocks before corrupting them.
- Use DBMS_REPAIR.FIX_CORRUPT_BLOCKS to mark the found problem
blocks as corrupt so that they will then signal ORA-1578
- If required use DBMS_REPAIR.SKIP_CORRUPT_BLOCKS to skip corrupt
blocks on the table.


(2) From Oracle 7.1 onwards you can use a ROWID range scan.
The syntax for this is a little tricky but it is possible
to select around a corrupt block using a ROWID hint.
As the format of ROWIDs changed between Oracle7 and Oracle8
there are 2 articles which discuss this:

Using ROWID Range Scans to extract data in Oracle8 and higher Note 61685.1
Using ROWID Range Scans to extract data in Oracle7 Note 34371.1


(3) If there is a primary key you can select table data via this index.
It may also be possible to select some of data via any other index.
This can be slow and time consuming and is only normally needed
for Oracle 7.0 releases. This method is described in Note 34371.1
(which also describes the ROWID range scans)

(4) There are various salvage programs / PLSQL scripts which can be
used to salvage data from a table. These can take longer to set
up and use than the above methods but can often cope with various
kinds of corruption besides an ORA-1578.
As these methods typically require much hand-holding from
support then some of these articles may not be visible to customers.

These require Pro*C to be available and an understanding of how
to build Pro*C executables:
SALVAGE.PC for Oracle7 Note 2077307.6

These requires manual interaction:
SALVAGE.SQL for Oracle7/8 Note 2064553.4


Methods of extracting data from a table with a corrupt LOBSEGMENT block

It is not possible to used DBMS_REPAIR on LOB segments.

If the corrupt LOB block is NOT referenced by any row in the table
then it should be possible to CREATE TABLE as SELECT (CTAS) or
export / drop / import the table as is.

If the corrupt LOB block is referenced by a row then it should be
possible to select or export with a WHERE predicate that excludes
the problem row/s.

WARNING:
It is possible to update the LOB column value
of a problem row to NULL which will then clear
ORA-1578 on SELECT operations *BUT* the corrupt
block will then be waiting to be reclaimed and will
eventually signal an ORA-1578 on attempts to get a
new LOB for INSERT or UPDATE operations on any row
which can be a worse situation than having a corruption
on a known row.
Hence you should only really set the LOB column to NULL
if you intend to immediately recreate the table.


Extracting data from the corrupt block itself

As the corrupt block itself is "corrupt" then any data extracted
from the block should be treated as suspect. The main methods
of getting the rows from the corrupt block itself are:

- For TABLE blocks Oracle Support can use a tool which attempts to
interpret the block contents.
- Use any existing indexes on the table to extract data for
columns covered by the index where the ROWID falls inside the
corrupt block. This is described towards the end of the ROWID
range scan articles mentioned above:
For Oracle8/8i see Note 61685.1
For Oracle7 see Note 34371.1

- It may be possible to use LogMiner on the redo stream
to find the original inserts/updates which loaded the
data to the problem block. The main factor here is
WHEN the data was actually put in the block.
eg; row 2 may have been inserted yesterday but row 1 may
have been inserted 5 years ago.



(4D) Leaving A Corruption In Place
It is possible to leave a corruption in place and just accept the
errors reported, or prevent access to the problem rows at an application
level.
eg: If the problem block / row is in a child table then it may be possible
at application level to prevent access via the parent row/s such that
the child rows are never accessed. (Be wary of cascade type constraints
though)

This may not help with reports and other jobs which access data in
bulk so it may also be desirable to use the DBMS_REPAIR options
shown in 4C above to prevent the block/s erroring when
accessed. Marking a corruption like this and leaving it around may
give a short term solution allowing full data salvage and/or recovery
to be attempted at scheduled outage, or allowing time to check other
recovery options on a second (clone) database. Note though that
marking a block corrupt with DBMS_REPAIR.FIX_CORRUPT_BLOCKS will cause
the marked block/s to also be corrupt after recovery through the
time that FIX_CORRUPT_BLOCKS was executed.

Leaving a corruption may be sensible for data which rapidly ages and is
subsequently purged (eg: In a date partitioned table where older
partitions are dropped at some point).


Leaving Corruptions in LOB segments

At application level it can be possible to leave a corrupt
LOB column in place until such time as the table can be rebuilt.

One way to ensure you do not hit the "WARNING" scenario above
is to ensure that the table is only ever accessed via a view
which includes a WHERE predicate to prevent the problem row/s
from being seen.
eg: Consider table MYTAB( a number primary key, b clob ) has
one or more rows pointing at corrupt LOB data.

ALTER TABLE MYTAB ADD ( BAD VARCHAR2(1) );

CREATE VIEW MYVIEW AS SELECT a,b FROM MYTAB WHERE BAD is null;

Set BAD='Y' for any problem row/s

If you only access MYTAB via MYVIEW and the row will never be visible
and so cannot be updated keeping the corrupt entry isolated until it
can be dealt with.

Clearly this example is more of a design time solution but some
applications may already have similar mechanisms and may only access data
via a view (or via an RLS policy) giving some option/s to hide the problem
row/s.


Warnings when Leaving a Corruption in Place
Whilst it is possible to leave a corruption in place it should be
noted that the corrupt blocks will still show up in runs of DBVERIFY,
in RMAN backup warnings / errors etc..

It is important to make a careful record of any corruption you
expect to see from these tools, particularly any blocks you expect
to skip with RMAN (eg: having MAX_CORRUPT set) and be sure to remove
any "acceptance" of the errors once the corruptions have been cleared.

eg: Consider that a corrupt block has been handled by leaving the corruption
in place and avoiding the problem row/s at application level.
RMAN may be configured to allow the corruptions during backup.
The table is then recreated at a later date during some table
reorganisation.
If RMAN is not updated to reflect that no errors should now be
expected then RMAN may ignore some other corruption which occurs
at a later time.

It is also important to note that leaving corrupt blocks around in
table segments can lead to mismatched results from queries
eg: different results can occur for tables with SKIP_CORRUPT set
depending on whether an index scan or table access occurs.
Other reports may just error .

Note that leaving a corruption in place but marking the block with
DBMS_REPAIR.FIX_CORRUPT_BLOCKS writes redo to corrupt the block
which may limit subsequent recovery options.



(4E) Last Options
If you have a standby setup (physical or logical) then check that first.

Whatever sort of block the problem occurred on, one possible option
is to recover the database, or problem tablespace, to a point in time
BEFORE the corruption appeared. The difficulty with this option is that
it is not always possible to know when the problem first appeared.

DBVERIFY can be often be used to check a restored file for corruptions.
For details of using DBVERIFY see Note 35512.1 . In particular the
START= / END= DBV options can be used to give a quick first test of whether
the problem block itself is bad on a restored backup image.

This section outlines some final options available for recovering.
If you have come here then one or more of the following have happened:

- You have lost a "vital" datafile (or have a corruption on it)
and have no good backup of the problem file/s (without the corruption)
- Are either not in ARCHIVELOG mode OR do not have all archivelogs
since the file was first created
- Complete recovery keeps reintroducing the problem


Last chance:
Please note if you have lost all copies of a datafile but DO still have
the ARCHIVE logs from when the file was first created it is still possible
to recover the file.
Eg:
ALTER DATABASE CREATE DATAFILE '....' [as '...'] ;
RECOVER DATAFILE '....'
ALTER DATABASE DATAFILE '....' ONLINE;

If you are in this scenario try to recover the datafile using these
steps before proceeding below.


If you have reached this line there are no options left to recover to
the current point in time. It is advisable to shutdown the instance and
take a BACKUP of the current database NOW in order to provide a fall-back
position if the chosen course of action fails. (Eg: if you find your backup
is bad).

Some outline options available are:

Revert to an old COLD backup
- eg: If in NOARCHIVELOG mode

Set up a clone database from a COLD backup
- and extract (export) the problem table/s
or transport the problem tablespace

Point in time recovery to an older point in time that is consistent
- requires a good backup and any necessary archive logs
- ALL files have to be restored and the whole DB rolled forward
to a suitable point in time.
- It may be possible to do the point in time recovery in a
clone database and then transport the problem tablespace
to the problem database, or export / import the problem table
from the clone to the problem database .

Tablespace point in time recovery
- It may be possible to perform a point in time recovery
of the affected tablespace only. There are many notes
describing tablespace point in time recovery such as
Note 223543.1.

Rebuild of DB from some logical export / copy
- Requires there to already be a good logical backup of the database
- NB: You have to RE-CREATE the database for this option.
- As with other options the rebuild could be in a clone database
just to get a good image of the problem table/s.


If you have a good backup then rolling forwards with DB_BLOCK_CHECKING=TRUE
can help find the first point in time where something started to go wrong.
It is not generally necessary to take the problem database down while
investigating the recovery options.
eg: You can restore the system tablespace and problem tablespace
datafiles only to a totally different location and/or machine
as a different instance to investigate how far you can roll forwards etc..
As of Oracle9i you can also use "Trial Recovery" options to save
having to keep restoring a backup while looking into your options.

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