SQL Server Documentation

Introduction

This document outlines critical considerations for implementing effective backup and restore strategies for Microsoft SQL Server. A well-defined and tested backup and restore plan is fundamental to data availability, disaster recovery, and business continuity. Understanding the various options and their implications is crucial for safeguarding your data.

Backup Strategies

Choosing the right backup strategy depends on your recovery point objectives (RPO) and tolerance for data loss. Common strategies include:

• Full Backups: Backs up the entire database, including the transaction log portion needed to recover the database. This is the foundation of any backup strategy.
• Differential Backups: Backs up all data that has changed since the last full backup. These are smaller and faster than full backups but require the last full backup to restore.
• Transaction Log Backups: Backs up the transaction log records since the last log backup. These are essential for point-in-time restores and are only available for databases in the FULL or BULK_LOGGED recovery models.

Restore Strategies

Restore operations are equally critical. The chosen restore strategy must align with your backup strategy and recovery time objectives (RTO).

• Restoring a Full Backup: The simplest restore, bringing the database back to the state it was in at the time of the full backup.
• Restoring a Full and Differential Backup: Requires restoring the last full backup followed by the last differential backup.
• Point-in-Time Restore: Achieved by restoring a full backup, followed by a differential backup (if used), and then a sequence of transaction log backups up to a specific point in time.

Recovery Models

The recovery model of a database significantly impacts backup and restore capabilities. Choose wisely based on your business requirements for data durability and recovery granularity:

• SIMPLE: Minimal logging. Transaction log space is automatically reused. Only full and differential backups are supported. Point-in-time restore is not possible.
• FULL: Logs all transactions. Supports full, differential, and transaction log backups. Enables point-in-time restore. Requires regular transaction log backups to prevent log file growth.
• BULK_LOGGED: Similar to FULL, but bulk operations (like BULK INSERT, SELECT INTO, CREATE INDEX) are minimally logged. This can improve performance for bulk operations but may impact restore capabilities if a log backup occurs during a bulk operation.

Note: Changing the recovery model from SIMPLE to FULL or BULK_LOGGED requires a full backup to be taken immediately afterward.

Backup Media

SQL Server can back up to various media:

• Disk: The most common and flexible option. Offers good performance and ease of management.
• Tape: Historically used for long-term archival, but disk-based solutions (like cloud storage or NAS) are often preferred now for speed and accessibility.
• URL (Azure Blob Storage): Allows backing up directly to Microsoft Azure Blob Storage, providing scalability and durability.

Consider the characteristics of your chosen media regarding performance, durability, cost, and accessibility for restore operations.

Backup Options

Several options can be specified during a backup operation:

• WITH COMPRESSION: Compresses the backup, reducing file size and potentially speeding up backup and restore operations, especially on systems with high I/O.
• WITH CHECKSUM: Verifies the integrity of the backup pages by calculating and storing checksums. This is highly recommended for detecting corruption.
• WITH DIFFERENTIAL: Specifies a differential backup.
• WITH COPY_ONLY: Creates a backup that does not affect the sequence of subsequent backups. Useful for ad-hoc full backups without disrupting the regular backup chain.
• WITH INIT / WITH NOINIT: Controls whether a backup overwrites existing backups on the media set or appends to it.

Example Backup Statements

Full Backup with Compression and Checksum:

BACKUP DATABASE [MyDatabase]
TO DISK = N'/var/opt/sqlserver/backups/MyDatabase_Full.bak'
WITH COMPRESSION, CHECKSUM, STATS = 10;
GO

Transaction Log Backup:

BACKUP LOG [MyDatabase]
TO DISK = N'/var/opt/sqlserver/backups/MyDatabase_Log.trn'
WITH COMPRESSION, CHECKSUM, STATS = 10;
GO

Restore Options

Restore operations also have crucial options:

• WITH NORECOVERY: Leaves the database in a restoring state, allowing further restore operations (like differential or log backups) to be applied.
• WITH RECOVERY: Rolls back any uncommitted transactions and makes the database ready for use. This is the final step in a restore sequence.
• WITH REPLACE: Allows overwriting an existing database with the same name. Use with extreme caution!
• WITH MOVE: Used when restoring a database to a new location or with different file names, allowing you to specify new paths for the data and log files.

Example Restore Statements

Restoring a Full Backup and bringing the database online:

RESTORE DATABASE [MyDatabase]
FROM DISK = N'/var/opt/sqlserver/backups/MyDatabase_Full.bak'
WITH RECOVERY, REPLACE, STATS = 10;
GO

Restoring a Full Backup and preparing for subsequent log restores:

RESTORE DATABASE [MyDatabase]
FROM DISK = N'/var/opt/sqlserver/backups/MyDatabase_Full.bak'
WITH NORECOVERY, REPLACE, STATS = 10;
GO

RESTORE LOG [MyDatabase]
FROM DISK = N'/var/opt/sqlserver/backups/MyDatabase_Log1.trn'
WITH NORECOVERY, STATS = 10;
GO

RESTORE LOG [MyDatabase]
FROM DISK = N'/var/opt/sqlserver/backups/MyDatabase_Log2.trn'
WITH RECOVERY, STATS = 10;
GO

General Considerations

• Backup Frequency: Determine how often backups should run based on your RPO. More frequent backups mean less data loss but can impact system performance.
• Retention Policies: Define how long backups should be kept for different purposes (e.g., daily, weekly, monthly, yearly archives).
• Storage: Ensure sufficient, reliable, and accessible storage for your backup files.
• Testing: Regularly test your restore procedures to ensure they work correctly and meet your RTO. A backup is useless if it cannot be restored.
• Monitoring: Implement monitoring for backup job failures and success.
• Security: Protect your backup files from unauthorized access or deletion. Store them in a secure location, potentially off-site.
• Database Size and Growth: Larger databases will require more storage and time for backups and restores.
• Transaction Log Size: In FULL or BULK_LOGGED recovery models, monitor the transaction log file size. If it's not being backed up regularly, it can grow uncontrollably, filling up the disk.

Best Practices

• Always include WITH CHECKSUM in your backup commands to detect corruption early.
• Consider WITH COMPRESSION to save storage space and improve performance where applicable.
• Perform regular, automated backups to disk.
• Keep at least one copy of your backups in a separate physical location (off-site or cloud storage).
• Document your backup and restore procedures clearly.
• Conduct periodic restore tests to validate your backup strategy and train personnel.
• Use SQL Server Agent jobs for scheduling and managing backups.
• Monitor backup job history and disk space used for backups.
• For critical databases, consider implementing backup verification using RESTORE VERIFYONLY or restoring to a test environment.

By carefully considering these factors and implementing robust backup and restore procedures, you can significantly enhance the resilience and availability of your SQL Server databases.