Log Shipping with Read-Only Standby

Log Shipping with Read-Only Standby in SQL Server: A Detailed Guide

Introduction

Log Shipping is one of the popular high-availability and disaster recovery solutions in SQL Server. It involves automatically sending transaction log backups from a primary database (also known as the primary server) to one or more secondary databases (also known as the secondary servers), where the log backups are restored.

One of the key features of log shipping is its ability to support a Read-Only Standby server, where the secondary server is not only used for disaster recovery but can also be used for read-only queries during the shipping process. This capability is particularly useful for offloading reporting and read-heavy workloads to the secondary server, thus improving overall system performance.

In this guide, we will cover Log Shipping with a Read-Only Standby, including all the steps, configurations, troubleshooting methods, and the benefits it provides in terms of performance, data protection, and scalability.

1. Understanding Log Shipping with Read-Only Standby

1.1 What is Log Shipping?

Log shipping is a process that involves the following steps:

Backup: A transaction log backup is taken periodically from the primary server.
Copy: The transaction log backup is copied to the secondary server.
Restore: The transaction log backup is restored on the secondary server.

This process allows a secondary server to be kept in sync with the primary server for disaster recovery purposes. In the case of a disaster on the primary server, the secondary server can take over, minimizing downtime and data loss.

1.2 The Concept of Read-Only Standby

In traditional log shipping setups, the secondary server remains idle except for the process of restoring the transaction logs. This means that the secondary server cannot be used for regular read/write operations. However, by enabling Read-Only Standby mode, the secondary database can be used to run read-only queries, thus offloading reporting or read-intensive workloads from the primary database.

Read-Only Standby Mode allows users to perform SELECT queries but prevents any modifications, such as INSERT, UPDATE, DELETE, or schema changes (DDL).
This makes it ideal for reporting or querying scenarios, where data is needed for analysis but the data should not change.

2. How Log Shipping Works with Read-Only Standby

2.1 Key Components of Log Shipping with Read-Only Standby

To configure log shipping with a read-only standby server, we need the following components:

Primary Server: The server where the production database resides and where transaction logs are backed up.
Secondary Server: The server where the transaction log backups are restored and optionally configured to allow read-only queries.
Monitor Server: A server that continuously monitors the status of the log shipping process and alerts administrators about failures.

2.2 Steps Involved in Log Shipping with Read-Only Standby

Log shipping involves the following steps to configure a Read-Only Standby server:

Set up Log Shipping: First, configure basic log shipping with the primary and secondary databases.
Enable Read-Only Mode on the Standby Database: After the secondary database is restored, configure it to be in Read-Only mode.
Monitoring: Configure the monitor server to check the health of the log shipping process.
Test the Read-Only Functionality: After configuring the read-only standby, ensure that users can execute SELECT queries but not modify data.

3. Setting Up Log Shipping with Read-Only Standby in SQL Server

Let’s go through the step-by-step configuration process for setting up log shipping with a read-only standby.

3.1 Step 1: Configure the Primary Database

Before enabling log shipping, ensure that the primary database is in Full Recovery Model. This recovery model is required for log shipping because it ensures that transaction logs are available to be backed up and shipped to the secondary server.

Steps:

In SQL Server Management Studio (SSMS), right-click the Primary Database.
Select Properties.
Under the Options page, set the Recovery Model to Full.

ALTER DATABASE [YourDatabase] SET RECOVERY FULL;

3.2 Step 2: Take a Full Backup of the Primary Database

Log shipping requires a full backup of the primary database to initialize the secondary database.

Right-click on the primary database in SSMS.
Select Tasks → Back Up.
Choose Full backup type.
Specify a destination for the backup file and click OK.

3.3 Step 3: Restore the Full Backup on the Secondary Server

On the secondary server, restore the full backup of the primary database. Make sure to restore it with the NORECOVERY option, so the database is ready to accept transaction log backups.

RESTORE DATABASE [YourDatabase] 
FROM DISK = 'FullBackupFilePath.bak' 
WITH NORECOVERY;

3.4 Step 4: Configure Transaction Log Backups

Next, create a transaction log backup on the primary server.

Take a Transaction Log Backup on the primary server by right-clicking the database and selecting Tasks → Back Up.
Choose Log as the backup type.
Specify a location to store the log backup file and click OK.

3.5 Step 5: Configure the Secondary Server to Restore Transaction Logs

On the secondary server, configure the transaction log restore process:

In SQL Server Management Studio, right-click on the secondary server database and choose Properties.
In the Transaction Log Shipping section, configure the following:
- Backup Folder: The location of the log backups from the primary server.
- Restore Delay: The amount of time the secondary server will wait before restoring the transaction logs.
- Restore Mode: Choose No Recovery mode for the initial restore.

3.6 Step 6: Enable Read-Only Standby Mode

Once the log shipping is configured and the transaction logs are being restored, you can enable Read-Only Standby on the secondary server.

To set the database to read-only mode:

Execute the following command on the secondary database:

ALTER DATABASE [YourDatabase] SET READ_ONLY;

After executing this command, the secondary database becomes read-only, and users can execute SELECT queries but cannot modify data.

3.7 Step 7: Configure the Monitor Server

The monitor server keeps track of the health and status of the log shipping process. This server can alert administrators in case of failures such as log shipping delays or failures to restore transaction logs on the secondary server.

To configure the monitor server:

On the primary server, go to SQL Server Management Studio.
In the Log Shipping section, specify the monitor server’s details.
Set up email notifications for failure alerts.

4. Benefits of Log Shipping with Read-Only Standby

4.1 Disaster Recovery and High Availability

Log shipping with read-only standby provides a disaster recovery solution, ensuring that a secondary database is always available to take over in case the primary database fails. By enabling read-only functionality on the secondary server, you can ensure the continuity of reporting and querying tasks, while the primary server can be restored to normal operation.

4.2 Offloading Read-Heavy Workloads

With the read-only standby feature, the secondary server can offload read-heavy workloads (such as reporting, business intelligence, and analytical queries) from the primary server. This can significantly reduce the load on the primary server and improve performance for transactional operations.

4.3 Reduced Latency for Reporting Queries

Read-only queries on the secondary server provide faster access to the data without impacting the performance of the production system. Reporting users or applications can query the secondary server without affecting the transactional workload on the primary server.

4.4 Cost-Effective Disaster Recovery

Log shipping with read-only standby provides a low-cost disaster recovery solution when compared to other high-availability solutions, such as Always On Availability Groups or Database Mirroring. It requires less overhead and can be set up using existing infrastructure.

5. Monitoring and Troubleshooting Log Shipping with Read-Only Standby

5.1 Monitoring the Log Shipping Process

SQL Server provides built-in tools to monitor the log shipping process:

SQL Server Management Studio (SSMS): Use the Log Shipping Status in SSMS to view the current status of the log shipping process, including the last transaction log backup, copy, and restore times.
Monitor Server Alerts: Set up email alerts in the monitor server to notify you of any failures in the log shipping process, such as missing transaction log backups or failures to restore logs.
Transaction Log Backup History: You can query the msdb.dbo.log_shipping_monitor views to get detailed information about the log shipping status and history.

SELECT * FROM msdb.dbo.log_shipping_monitor
WHERE primary_database = 'YourDatabaseName';

5.2 Troubleshooting Log Shipping Failures

If you encounter issues with log shipping, such as the transaction log not being restored on the secondary server, follow these steps:

Check the SQL Server error logs for any error messages related to the transaction log restore process.
Verify the permissions for the user account responsible for copying and restoring the transaction log backups.
Ensure that the log shipping jobs are running successfully by checking the SQL Agent Jobs on both the primary and secondary servers.
Verify the network connectivity between the primary and secondary servers, especially if the log shipping jobs are failing to copy log backups.

6. Best Practices for Log Shipping with Read-Only Standby

Regular Backups: Ensure that you take regular backups on both the primary and secondary servers to protect against data loss.
Monitor Transaction Log Shipping: Continuously monitor the status of log shipping jobs and alerts to identify potential failures early.
Use Reliable Backup Storage: Store backup files in a reliable, secure location with sufficient disk space.
Network Bandwidth: Ensure that you have sufficient network bandwidth between the primary and secondary servers for efficient log shipping.
Test Failover Procedures: Periodically test failover procedures to ensure that the secondary server can take over as expected in case of a disaster.

Log shipping with Read-Only Standby in SQL Server provides a powerful, cost-effective disaster recovery solution while enabling offloading of read-heavy workloads. By following the steps for configuration, monitoring, and troubleshooting, you can ensure that your SQL Server environment is highly available and performant.

The read-only feature on the secondary server allows you to run reporting and analytical queries without affecting the performance of the primary database, ensuring business continuity and improved scalability. The combination of transaction log shipping and read-only standby offers an excellent balance between high availability, cost-effectiveness, and performance in SQL Server environments.