Pages

Monday, 7 February 2022

SQL Server – Backing up the Tail of the Log

When a database gets corrupted or failure occurs, before restoring the database from the backups you've created, it is recommended to create a tail-log backup of the records that haven't been backed up. This helps restore the database to the exact point at which it failed, preventing data loss.

Read on to learn about the other reasons when you need to back up the tail of the transaction log. Also, understand how to take a tail-log backup and restore it to get back the data you fear losing in the event of a crisis.

Why and When Should You Back Up The Tail of the Log?

Tail-log backup helps capture the tail of the log records when the database is offline, damaged, or data files are missing.

Reasons Why You Need To Back Up the Tail of the Transaction Log

  • Database is corrupted, or the data file is corrupted or deleted.
  • Database goes offline and doesn't start; you may want to recover the database as quickly as possible. But, before you begin recovery, first take the tail-log backup.
  • The database is online, and you plan on restoring the database, start by backing up the tail of the log.
  • Migrating database from one server to the other server.

Example Demonstrating the Need to Take Tail-log Backup

Let's say, you run DBCC CHECKDB to check for corruption in the database. It returns consistency errors and you decide to restore the previously taken backups, such as the Full backup. Then, you restore the Differential and all the transaction log backups. But you don't want to lose the log records that haven't been captured in the transaction log backup. So, to avoid losing those log records (i.e., the tail of the backup) and maintain the log chain intact, you will need to take Tail Log Backup.

Let’s consider a scenario.

Assume taking a Full database backup and Log backups after every one hour.

Time

Event

8:00 AM

Create a Full Database Backup

9:00 AM

Take Transaction Log Backup

10:00 AM

Take Transaction Log Backup

11:00 AM

Take Transaction Log Backup

11:30 AM

Failure occurs


You can restore the database starting from Full backup (taken at 8 AM), then restore all the three transaction log backups (taken at 9 AM, 10 AM, 11 AM). But, there are no backups from 11:00 AM till 11:30 AM, resulting in data loss.

So, how to recover the database without data loss between 11-11:30 AM?

Take t-log backup by executing the BACKUP LOG command ‘WITH NO_TRUNCATE’ option. It will create a t-log backup file. Restore the file after the last transaction log backup (11 AM) WITH NORECOVERY to recover the lost data.

USE MASTER

GO

BACKUP LOG [Database] TO DISK = ‘C:\ProgramFiles\MSSQLServer\Data\Tail_Log1.LOG’ WITH NO_TRUNCATE;

How to Back up and Restore Tail of the Log?

Before we discuss the process to back up the tail of the transaction log and restore it, it’s important to know the clauses you need for creating a t-log backup.

  • NORECOVERY: Using this clause leave the database into the restoring state. This assures that the database won’t change after the t-log backup.
  • NO_TRUNCATE: Use this clause only when the database is damaged.

  • CONTINUE_AFTER_ERROR: If a database is damaged and you cannot take t-log backup, back up the tail of the log using CONTINUE_AFTER_ERROR.

Demo

  • Create a new database

CREATE DATABASE Tail_LogDB;

GO

USE Tail_LogDB;

GO


  • Create a new table and insert some data into it.

CREATE TABLE Employee (

EmployeeID int IDENTITY(1000,1) PRIMARY KEY NOT NULL,

EmployeeAge int

);

GO

This T-SQL query will create a table named Employee with columns ‘EmployeeID’ and ‘EmployeeAge’. 


  • Create a stored procedure to add more records to the table.

CREATE PROCEDURE InsertEmployee

AS

DECLARE @i int = 100

WHILE @i > 0

BEGIN

INSERT Employee (EmployeeAge) VALUES (@i)

Set @1 -=1

END

GO

EXECUTE InsertEmployee;

GO

SELECT * FROM Employee;

GO

Executing this T-SQL query will create an ‘InsertEmployee’ stored procedure that runs through a loop to add 100 more records into the Employee table. Then, select the Employee table to verify that everything works.


  • Create a full backup of the Tail_LogDB

BACKUP DATABASE Tail_LogDB

TO DISK = 'C:\TempDB\ Tail_LogDB_FULL.bak'

This command will create a full database backup with the 100 records we added in the table in Step 3. And, the backup gets saved in a folder we have created, ‘TempDB’.

  • Insert some more records into the table

EXECUTE InsertEmployee;

GO

SELECT * FROM Employee;

GO

After executing this T-SQL query, we will have 200 records in the database table. 



  • Simulate a database failure

If you're keeping your data and log files on different physical drives, then it's entirely possible that drive failure takes out the data file and leaves you only with the transaction log. We can simulate this simply by deleting the mdf file from the hard drive. Here's how:
  • Right-click on Tail_LogDB > Tasks > Take Offline. 


  • Select the ‘Drop all active connections’ checkbox and press OK. 


  • Now refresh the database, and you can see that the db is now OFFLINE. 

  • Next, go to the location where the data file is stored (i.e., TempDB folder), and you can see the db that we just saved. 


  • Now go to the location where the .mdf file and .ldf files for the Tail_LogDB database are saved. Delete the .mdf file.  


Now let's head back to SSMS and understand how we can recover from this disaster.

Bring Database Back Online

·         Right-click on Tail_LogDB > Tasks > Bring Online.


·         A dialog box with errors, click Close.


         Refresh the database.

As you can see, the database status has changed to Recovery Pending. Before attempting the restore operation, ensure to back up the tail of the log to capture the second instance of the 100 records we added into the database.

Now, let’s take the tail of the log. 

Switch into the master database and execute the BACKUP LOG statement with the CONTINUE_AFTER_ERROR option. This option will ensure to perform tail log backup even if any error occurs.

USE master;

GO

BACKUP LOG Tail_LogDB

TO DISK = 'C:\TempDB\Tail_LogDB.log'

WITH CONTINUE_AFTER_ERROR;

GO

Restore the t-log backup

Let's initiate the restore process by restoring the full database backup 'WITH NORECOVERY' option. Using this option specifies that the restore procedure would not attempt to undo or roll back any uncommitted transactions. This is important because if a modification to the data had begun but not finished when the failure occurred, there would be a record in the transaction log. Typically, SQL Server will attempt to roll back any of these partially completed changes during a restore, and we don't want this to happen.

USE master

RESTORE DATABASE Tail_LogDB

FROM DISK = 'C:\TempDB\Tail_LogDB_FULL.bak'

WITH NORECOVERY;

GO

This restores the backup of the first 100 records. 


To complete restoring the entire record set, let's restore the log file as well.

RESTORE LOG Tail_LogDB

FROM DISK = 'C:\TempDB\Tail_LogDB.log';

GO


·         Verify the results

USE Tail_LogDB

SELECT * FROM Employee;

GO


So, as you can see, all the 200 records are now restored.

Conclusion: Key Take-Away Points

  • A tail-log backup is useful to avoid losing data when a database is damaged or corrupted. However, you may fail to back up the tail of a damaged database log. So, when executing the BACKUP LOG statement, use WITH CONTINUE_AFTER_ERROR option to take t-log backup.
  • You must also take a tail-log backup before restoring a database in an ONLINE state. If the database is in OFFLINE state and doesn't start, back up the tail of the transaction log WITH NORECOVERY before performing the restore procedure.
  • It is also recommended to take t-log backup when migrating a large database from one source to another.
  • But remember, you can take tail-log backups only if the transaction log file is accessible. Meaning, you cannot perform t-log backup on a database with a corrupted and inaccessible log file.