Understanding Microsoft SQL Server Transaction Log Simplified


SQL Server Transaction Logs | Hevo Data

The volume of data that businesses collect has grown exponentially in recent years. This is primarily due to the fact that businesses today rely on data-driven decision-making like never before. For obvious reasons, it is of paramount importance to these businesses that their data is not lost in any way. Most modern databases house features to ensure that user data is not lost. One of the most well-known and robust implementations to ensure no data loss is by Microsoft in SQL Server Transaction Logs.

This article will help you understand what Microsoft SQL Server is, what Microsoft SQL Server Transaction Logs are and how they can help you ensure that you never lose your data.

Table of Contents

What is Microsoft SQL Server?

Microsoft SQL Server is a well-known Relational Database Management System (RDBMS). Considering it is an RDBMS, its primary purpose is to store and retrieve data as per the requirements of the users and the applications connected to it. Microsoft SQL Server provides an implementation for a wide range of functionalities including Transactional Processing, Business Intelligence, Data Analytics, etc. that are required by most businesses today. It is considered to be one of the top three leading Database Technologies along with Oracle Database and IBM DB2.

Along with support for ANSI SQL, which is the popular Structured Query Language (SQL), Microsoft SQL Server comes with its own implementation of SQL called Transact SQL (T-SQL). T-SQL supports additional capabilities that allow users to declare variables, handle Exceptions, etc. The main interface tool for Microsoft SQL Server is called SQL Server Management Studio (SSMS).

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What are the Key Features of Microsoft SQL Server?

The key features of Microsoft SQL Server are as follows:

1) Accelerated Data Recovery

There might be situations in which a complex transaction being run on the Microsoft SQL Server fails or the Server crashes while performing an operation. In these situations, the database has to be recovered to its initial state by rolling back all incomplete transactions and operations. This process was considered to be time-consuming until Microsoft’s implementation of Accelerated Data Recovery in Microsoft SQL Server.

The process that performs database recovery in Microsoft SQL Server has been redeveloped using Transaction Logs which has led to a drastic decrease in the time taken.

2) Encryption

Considering that databases store sensitive information for most businesses, it is essential for Database Providers to ensure that their databases are secure. Microsoft SQL Server 2016 introduced a new Encryption Technology called AlwaysEncrypted that allowed transparent Column Encryption without giving Database Administrators access to Decryption Keys.

3) Intelligent Query Processing

Microsoft SQL Server houses a robust in-built Query Optimizer that generates an execution plan for the queries being executed. Various improvements have been made to this Query Optimizer to ensure that the databases provide the best performance to their users. These improvements include Dynamic Memory Grants for Rowstore Tables, Batch Mode on Rowstore, Table Variable Deferred Compilation, etc.

4) Advanced Analytics

All data stored in Microsoft SQL Server can be easily leveraged to perform a comprehensive analysis as per requirements using Microsoft SQL Server Analysis Service (SSAS) which is a Data Mining and an Online Analytical Processing tool in Microsoft SQL Server.

5) Advanced Integrations

Microsoft SQL Server also allows users to perform a broad range of tasks such as Data Extraction, Transformation, Loading, Data Migration, etc. using the Microsoft SQL Server Integration Services (SSIS). SSIS can be used to extract and transform data from various sources such as Relational Databases, Flat Files, XML Files, etc. using its Graphical User Interface (GUI) without writing a single line of code.

What are ACID Properties?

In order to understand what Microsoft SQL Server Transaction Logs are, you first need to understand what ACID properties are and why they’re important.

In the concept of RDBMS, the ACID Properties refer to four key properties of every transaction that takes place in the database. These properties are as follows:

  • Atomicity: Any changes that are being performed to the database are performed in a single operation. This means that either all changes are made or none of them are. For example, in a Banking Transaction, Atomicity ensures that if money is debited from the sender’s account, it is deposited in the recipient’s account as well.
  • Consistency: Ensures that the database stays consistent before and after a transaction has taken place. For example, in a Banking Transaction, if money is being transferred from one account to another, Consistency ensures that the total amount of money in both the accounts before and after the transaction is the same.
  • Isolation: Ensure that multiple transactions can run concurrently on a database in a serialized form. For example, in a Banking Transaction in which money is being transferred from Account ‘A’ to Account ‘B’ in one transaction, if another parallel transaction tries to check the balance of Account ‘A’ and Account ‘B’, it will see the funds being transferred in only one of the accounts and not in both or neither of them.
  • Durability: Once the transaction is complete, the changes cannot be reversed even if the system fails. For example, in the Banking Transaction, there is no way to reverse a transfer of money from one account to another.

What is a Transaction Log?

Every SQL Server database includes a transaction log, which is a file. It stores log records generated by the SQL Server database logging process. When it comes to disaster recovery, the transaction log is the most critical component of a SQL Server database – but it must be undamaged. The transaction log is updated after each database modification – transaction occurrence. All of the modifications are made in order.

What SQL Server Transaction Log store?

Except for those that are minimally logged, such as BULK IMPORT or SELECT INTO, a transaction log stores every transaction made to a SQL Server database. Internally, it is divided into Virtual Log Files, which are smaller sections (VLFs). Continue writing to the next available VLF in the transaction log when one VLF becomes full. A circular file can be used to represent the transaction log. When the logging reaches the end of the file, it restarts from the beginning, but only if all of the requirements have been met and the inactive sections have been removed. To mark all inactive parts so they can be used and overwritten again, the truncation process is required.

If all of the following are true, a log record in the transaction log is no longer required:

  • It is a part of a transaction that has committed.
  • A checkpoint has written all of the database pages it changed to disc.
  • A backup does not require the log record (full, differential, or log)
  • Any feature that reads the log does not require the log record (such as database mirroring or replication)
  • The transaction log’s logical log is an active component. Every transaction in the transaction log is identified by a Log Sequence Number (LSN). The MinLSN is the beginning of the online transaction log’s oldest active transaction.

What is Microsoft SQL Server Transaction Log?

A Microsoft SQL Server Transaction Log is an integral part of every SQL Server Database. The basic idea behind a Microsoft SQL Server Transaction Log is that it is used to maintain a record of every operation that is performed on the database in the order in which it was performed. This information is used in case the transaction fails or the Server crashes while an operation is being performed in order to recover the database to its original state. Hence, Microsoft SQL Server Transaction Log is considered to be the most important component for Disaster Recovery.

All Relational Database Management Systems (RDBMS) are expected to be ACID compliant and Microsoft SQL Server Transaction Logs is how SQL Server ensures all its databases are ACID compliant.

A Microsoft SQL Server Transaction Log is internally split into smaller parts called Virtual Log Files (VLFs). All logs are written to these smaller VLFs. Once one VLF becomes full, new logs are written to the next available VLF in the Microsoft SQL Server Transaction Log file. The Microsoft SQL Server Transaction Log can be seen as a circular file. This means that when the logging reaches the end of the file i.e. when the log file is full, the logging process starts again at the beginning of the file from the first VLF.

An observation can be made that if the logging takes place in a circular file, it means that old logs at some point will get overwritten by new logs which means the old logs will have to be deleted. The old logs can be deleted if all of the following conditions are met:

  • The corresponding transaction for which the log is about to be overwritten is complete.
  • All changes made by the corresponding transaction have been written to disk by a checkpoint.
  • The log is not required for backup.
  • The log is not required for performing any operation that accesses the log. For example, Database Mirroring or Replication.

An example of a Microsoft SQL Server Transaction Log file is as follows:

A Log Sequence Number (LSN) is used to uniquely identify each transaction in the log. The Logical Log represents the active part of any Transaction Log and MinLSN can be seen as the starting point of the oldest active transaction in the log.

The various operations supported by Microsoft SQL Server Transaction Logs are as follows:

  • Recovery of Individual Transactions.
  • Recovery of Incomplete Transactions upon Startup.
  • Perform Database Replication.
  • Support High Availability and Data Recovery Solutions such as Database Mirroring, Log Shipping, etc.

What are the Microsoft SQL Server Recovery Models?

Each Microsoft SQL Server database includes a property called the Recovery Model that helps determine how transactions are logged, whether the Microsoft SQL Server Transaction Logs can be backed up or not, and the type of restore options available. 

A Microsoft SQL Server Database can have any of the following Recovery Models:

1) Simple Recovery Model

Backups for Microsoft SQL Server Transaction Logs are not supported in this model. This means that you do not have to handle the overhead associated with maintaining Transaction Log backups. The Log Space is automatically reclaimed in this model and hence, there is no need for users to manage the Transaction Log Space. 

There are however certain disadvantages associated with this model. The biggest disadvantage is that it can only restore the database to the point of its last backup. Hence, all transactions that were performed after the most recent backup are lost. The Simple Recovery Model is, thus, considered to be suitable only for Development and Test databases, or in Application databases if data loss is acceptable.

2) Full Recovery Model

In this Recovery Model, all transactions are fully logged into the Microsoft SQL Server Transaction Log file. The Log Sequence is unbroken and is preserved for any possible Database Recovery Operations. Transaction Log files can be backed up in the model and hence, it provides full recovery to any point. Therefore, this model is considered to be less risky than the Simple Recovery Model. This Recovery Model should be used for supporting mission-critical applications to ensure that there is minimal or no data loss.

3) Bulk Logged Recovery Model

This is a special configuration option that is quite similar to the Full Recovery Model. The only difference between this model and the Full Recovery Model is that this model allows some operations to be minimally logged. The technique used by Microsoft SQL Server Transaction Log files to log bulk operations is called Minimal Logging. This model should be used when the database is subjected to bulk operations on a regular basis and the growth of the Microsoft SQL Server Transaction Log file has to be prevented.

The Recovery Model for a database can be changed simply using the ALTER DATABASE command. The query to perform the operation is as follows:

ALTER DATABASE {db_name} SET MODEL {model_name};

{db_name} in the above query has to be replaced with the name of the database for which the Recovery Model is being set and {model_name} has to be replaced with SIMPLE, FULL, or BULK_LOGGED based on requirements.

The Recovery Model can also be changed using Microsoft SQL Server Management Studio (SSMS) by opening the SQL Instance in the Object Explorer, selecting the required database, and changing the Recovery Model in the properties.

Can SQL Server database work without a Transaction Log?

No, due to SQL Server’s design and ACID compliance, this is not possible. Atomicity, consistency, isolation, and durability are the characteristics of ACID. All transactions must meet the following criteria:

  • An atomic transaction is either completely completed or never started.
  • A transaction ensures that the system state is consistent by ensuring that the system is in a valid state at the end of each transaction.
  • When a transaction runs in isolation, it appears to be the system’s only action at any given time.

Can one SQL Server database have more than one Transaction Log?

Yes, that is possible, but it is only advised in certain circumstances. Adding multiple transaction log files to a SQL Server database will not improve performance in any way. Because only one file can be written to at a time, parallel I/O operations are not possible.

If the first transaction log file is full or the disc drive is running out of space, having multiple transaction log files is recommended. In any case, these issues should be addressed sooner rather than later by creating transaction log backups and monitoring disc drive space availability.

Monitoring disk space used by SQL logs

Why is the SQL Server Transaction Log growing?

Logging into the online transaction log follows each transaction. Because the transaction log grows as changes are made to a database during SQL Server work, keeping it up to date is critical for proper SQL Server operation.

In SQL Server, there are three recovery models, and transaction log growth manifests differently depending on which one is used:

  • Simple recovery model: Backups of transaction logs are not possible. The process of truncation is automated, and space is reclaimed for future use. Because changes since the most recent database backup are exposed, there is a risk of data loss. There is little chance of the transaction log growing in the Simple recovery – unless there is a long-running transaction or a transaction that creates many changes.
  • Bulk-logged recovery model: Backups of transaction logs are supported and required on a regular basis. Because there is no automated transaction log truncation, regular transaction log backups are required to mark unused space for overwriting. The bulk-logged recovery model uses minimal logging for most bulk operations, reducing transaction log space usage.
  • Full recovery model: Backups of transaction logs are supported and required on a regular basis. Under normal circumstances, there is no risk of data loss. Because there is no automated transaction log truncation, regular transaction log backups are required to mark unused space for overwriting. Because all transactions are logged, the transaction log is most likely to grow during a full recovery.

How to Maintain Microsoft SQL Server Transaction Log Files?

The maintenance of the Microsoft SQL Server Transaction Log is an important task to ensure the overall maintenance of the database. The Transaction Log Space has to be monitored regularly. If the database has a lot of users, this monitoring has to be done on daily basis.

Information on the Transaction Log Space can be monitored using the following query: 


Backups of the Microsoft SQL Server Transaction Log file should be made regularly. The Transaction Log file should be cleared every time a backup is made to ensure that no Auto Growth Operations take place.

Backup of the Transaction Log of a database can be made using the following command:

BACKUP LOG {db_name}
TO DISK = “D:{db_name}.trn’

{db_name} in the above query has to be replaced with the name of the required database.

Do we need SQL Server Transaction Log backups?

Yes, one of the most important resources for disaster recovery is the Internet. They are only required (and available) if the Simple recovery model is used; however, there is a risk of data loss. For high-traffic SQL Server databases, the majority of Database Administrators use a 15-minute interval or even less. Backups of transaction logs are important because they identify inactive VLFs that can be used to record new transactions.

How to Truncate and Shrink Transaction Log files in SQL Server?

Understand truncating the log

When a database uses a simple recovery model, the system automatically truncates the transaction log after each transaction. The system truncates the transaction log only after a successful transaction log backup for databases with a full or bulk-logged recovery model.

The log is not truncated by a full database backup. If you don’t back up your transaction logs on a regular basis, the log file will grow until it runs out of space. Although truncating the log frees up space, it does not reduce the size of the transaction log file. You must shrink the transaction log file in order to reduce its size.

Truncate the transaction log

To truncate the transaction log file in SQL Server Management Studio, follow the steps below (SQL Server 2008 and later). It’s important to remember that following these steps could result in data loss. You shouldn’t have to truncate the log manually because regular log backups should take care of it.

  • Select Properties -> Options from the right-click menu of the database.
  • Exit the menu after changing the recovery model to Simple.
  • Select Tasks -> Shrink -> Files from the right-click menu of the database.
  • Change the log type.
  • Select Reorganize pages before releasing unused space under Shrink action and click OK.
  • Switch the recovery model back to Full or Bulk-Logged when the process is finished, and take a full database backup.

Understand shrinking the log

Consider shrinking the transaction log file if you need to free up some disc space. Data is moved from the end of the file to the unoccupied space at the front of the file when shrinking. It can be deallocated and returned to the file system once the process has created enough space at the end of the file. After you perform an action that generates a large number of logs, shrinking logs can help. Only free space on the log file allows you to shrink the log.

Shrink the transaction log

  • To truncate the transaction log file, follow these steps:
  • Select Tasks -> Shrink -> Files from the right-click menu of the database.
  • Change the log type.
  • Select Release unused space under Shrink action and click OK.


This article provided you with an in-depth understanding of what Microsoft SQL Server Transaction Logs are, why they’re important, what are the various Recovery Models used for the databases, and how you can maintain the Transaction Log for your Microsoft SQL Server database.

Most businesses today use multiple platforms to carry out their day-to-day operations. As a result, all their data is spread across the databases of these platforms. If a business wishes to perform a common analysis of their data, they would first have to integrate the data from all these databases and store it in a centralized location. Building an in-house data integration solution would be a complex task that would require a high volume of resources. Businesses can instead use existing data integration platforms like Hevo.


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Manik Chhabra
Former Research Analyst, Hevo Data

Manik has a keen interest in data, software architecture, and has a flair for writing hightly technical content. He has experience writing articles on diverse topics related to data engineering and infrastructure. The problem solving and analytical thinking ability combined with the impact he can make in data professional's day to day life motivate him to create content.

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