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DBCC CHECKDB

14 November,2011 by Jack Vamvas

Torn Pages \ Corrupt pages – all in a days work for a  DBA. But can anyone tell me how to use DBCC CHECKDB and the steps to take if a corrupt page is discovered? 

This is a typical question I’ll ask at an interview for a DBA. In most cases the answers are vague .

My approach on DBCC CHECKDB.

The main function of DBCC CHECKDB is to check consistency errors.

DBCC CHECKDB ('yourDB') WITH NO_INFOMSGS  is the basic command. Running this command will commit all the integrity checks

Use DBCC CHECKDB (‘MyDB’) WITH PHYSICAL_ONLY to check just the physical consistency of the database. This is a faster option

If errors are found , the first step is to assess the situation and check your backups. You don’t have a backup?  Make a note and ensure you implement a valid backup strategy in the future.

Maintaining a valid backup policy is the best way I’ve found to deal with integrity errors

An example of output is :

 

The Index Allocation Map (IAM) page (0:0) is pointed to by the previous pointer of IAM page (1:913345) in object ID 0, index ID -1, partition ID 0, alloc unit ID 290336478199808 (type Unknown), but it was not detected in the scan.
CHECKDB found 1 allocation errors and 0 consistency errors not associated with any single object.
CHECKDB found 1 allocation errors and 0 consistency errors in database 'yourdb'.
repair_allow_data_loss is the minimum repair level for the errors found by DBCC CHECKDB (yourDB).

 

In this example the corrupted page is (1: 913345). Is it a heap, non-clustered index or clustered index?

 Examine the page using this method:

 

DBCC TRACEON (3604, -1)
GO
DBCC PAGE('yourdb', 1, 913345, 3)
GO 

 

In the output you will see something like:

Metadata: IndexId = n

If n is greater than 1 it is a non-clustered index and can safely be dropped and recreated. If n is 0 or 1 you have data corruption and need to perform one of the options described below.

What should I do ?

The cleanest method of recovering is the last valid backup. If  the Recovery Model is Full , then RESTORE the last valid backup.

If LOG backups are maintained – then a) backup the tail b) RESTORE the valid backup with NO RECOVERY , c) Restore the Log files d) Restore the tail

If the Recovery Model is Simple , then you can only recover the last known full backup

Repair Options

1)    Do NOT proceed beyond this point , without first taking a FULL SQL Server database backup. One option is a Copy only backup in SQL server 2005 and not break the backup chain

2)    Now that you have a FULL backup , let’s proceed with the Repair Options. You’ll notice in the output above a message “xxxxxx is the minimum repair level for the errors found by DBCC CHECKDB” at the bottom of the message.  

This message specifies the minimum repair level

3) If you’re lucky the message specifies REPAIR_REBUILD. This option commits the repairs with no possibility of data loss.

The syntax for REPAIR_REBUILD is :

DBCC CHECKDB('yourDB', REPAIR_REBUILD)

4) If you’re less lucky you’ll see REPAIR_ALLOW_DATA_LOSS . There is a possibility of data loss during these repairs:

 DBCC CHECKDB('yourDB', REPAIR_ALLOW_DATA_LOSS)

After you’ve completed one of these repair options – run the CHECKCONSTRAINTS integrity checks. The example syntax checks all enabled and disabled constraints on every table on the database  :

 DBCC CHECKCONSTRAINTS WITH ALL_CONSTRAINTS;
 

6)   A more conservative but safer method for table corruption is to use DBCC CHECKTABLE. This checks the integrity of the tables. If you identify a corrupt table, one approach may be to attempt a batch by batch export of data into another table, until you hit the problem area.

Summary

Always have a valid backup available. In most cases , this remains your best solution.

Before you commit to the repair methods , list all possible methods of alleviating risk.

 

Author: Jack Vamvas (http://www.sqlserver-dba.com)

Author: Jack Vamvas (http://www.sqlserver-dba.com)

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