4/26/2016

How to Recover Data From Corrupted Log File In SQL Server ?

Introduction

SQL Server is a database platform designed for providing large-scale transactions, e-commerce applications, data mining, etc. It is widely used on Business platforms for data analysis, data integration and processing components.

Structured Query Language (commonly known as SQL) is a programming language used for managing data held in relational database management system (RDMS) consisting of data definition language, data manipulation language and a data control language. However, the SQL server database consists of three files:

  1. Primary Database File (MDF file)
  2. Secondary Database File (NDF file)
  3. Log file (LDF file)

Among these three, one database file is Log File. The file maintains a log of all the transactions done in SQL Server Database so that information can be later accessed to recover the database. There must exist one log file for each database and it is possible that more than one log file can be created for a single database. The file is in .ldf file extension.

How SQL Server Log File Corrupt?

The log files of the SQL server may be corrupted, i.e. while performing some action upon SQL database, an interruption in between the procedure is probable to occur due to which user first has to repair and recover the log file corrupted SQL server and then continue their transaction.

Causes of Log File Corruption

  • Viruses or other Malicious Software: In computer system, many viruses can infect and damage the log files and makes them inaccessible.
  • Terminating System Abnormally: If system/application is quit abnormally, then files are prone to be corrupted or damaged.
  • Input Output Configuration: The I/O subsystem is a vital component of database used to store system and user databases. Hence if the configuration is disturbed on enhanced that it may lead to corruption in log files
  • Storage Size Issue: The biggest reason behind the corruption of log files is the storage size. In case the data exceeds the limit of LDF, corruption is likely to occur.

How to Repair Corrupt SQL Log File ?

We will be dealing with corrupted log file for repairing it and recovering data from it by executing commands in SQL server as a hit and trial method. User may apply any one the following workaround and if none of the technique resolve the problem then it means that log file is highly corrupted and can only be repaired using a third party tool.

Solution 1: Using Backup of Log (LDF) File

If users create the backup of log file after every transaction, then they can easily retrieve their data from that backup file.

Solution 2: If Event Log Files are Corrupted

If event log files are corrupted, follow the following steps:

  1. Right-Click on the system folder.
  2. From the menu displaying, select on Properties and then click on Clear Log.
  3. A warning box will display that do you want to archive the system log files, click Yes to continue.
  4. Now save the file anywhere on your system.
  5. Now you will be back to properties window, in that click on OK option.
  6. Close and reopen the application window to see the changes

Solution 3: Put Database Online & Offline

If crash recovery function is enabled, then the database in online mode leads to transaction roll back. Follow the following steps:

  1. Put the database in an offline mode.
  2. Clear active log by using checkpoint to delete corrupted log files.
  3. Now again pitch to Online mode and try to access log files.

Solution 4: Recover using DBCC CHECKDB

In order to repair a corrupt LDF file, use the WITH TABLOCK option for DBCC CHECKDB. It will recover the data from a corrupted LDF file that has been corrupted or damaged due to some reason such as logical corruption.

Solution 5: Recover using CHECKDB Command:

For rebuilding log files, we use another technique, i.e. putting database into Emergency mode and then executing CHECKDB command. However, to change the database mode execute following command:

ALTER DATABASE dbname SET EMERGENCY, SINGLE_USER

Similarly, for repairing the log file using CHECKDB command execute the following syntax:

DBCC CHECKDB(‘dbname’ , REPAIR_ALLOW_DATA_LOSS)

Bottom Lines

Here we end up with the fact that manual procedure has a limitation and the technique does not give guarantee the outcome of an exact solution for repairing the log transaction file corrupted in SQL server, it’s just a hit and trial method. However, another drawback is that it is too complex and if something goes wrong then the result may be complete database loss. Therefore, the most reliable and trouble free method is to use SQL Server Transaction Log Recovery Tool, which repair and recover data from corrupted log files. The data integrity is maintained and user can export LDF files into the featured output format for reading the entries without dependency on server.

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