Tuesday, October 18, 2011

Reasons for LOB data corruption and steps to perform MDF File recovery

Most of the time, we face situations wherein we need to store large amount of data in an SQL Server table. Data isn’t always limited to characters, strings, or numbers. Large data can be categorized as documents, photos, videos, raw files, etc. SQL uses advanced data types for storing bulk of data. LOB data types generally include BLOB (Binary LOB), CLOB (Character LOB), and DBCLOB (Double-byte Character LOB). But, LOB data structures are prone to corruption. As a consequence, all valuable data stored in your SQL database becomes inaccessible. To overcome this problem, you need to go for SQL database recovery through a reliable third-party utility.

For instance, when you run a query that references LOB data, you may come across the following error message on your screen:

 “Msg 7105, Level 22, State 6, Line 1
The Database ID 11, Page (255:177), slot 1 for LOB data type node does not exist. This is usually caused by transactions that can read uncommitted data on a data page. Run DBCC CHECKTABLE”

Any of these reasons might have caused the above error message:
  • The query that you ran uses the ‘READ UNCOMMITTED ISOLATION LEVEL’ or the ‘NOLOCK’ query hint.
  • There is corruption in one of the database pages or the referenced LOB page structure.
  • An application or database engine error.
This could also lead to complete database inaccessibility. Try using an updated backup to resolve the issue. However, if the backup is unavailable, you should follow the below mentioned resolution steps.

The following steps may help you fix the error:
  • You can try using DBCC CHECKDB command to fix logical corruption in the database.
  • If DBCC CHECKDB indicates no problems, rerun the query. 
  • If the problem still persists, download the latest service pack for MS SQL Server.
If the above resolution steps are unable to resolve the issue, take help of efficient SQL database recovery software. These software encapsulate advanced mechanisms to effectively repair corrupt SQL databases and retrieve all precious objects in any SQL database corruption scenario.

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