SQL Server Database Engine Recovery Models

Understanding and correctly configuring the recovery model for your SQL Server databases is crucial for managing data integrity, recoverability, and performance. The recovery model determines how transactions are logged, which operations are logged, and what types of restore operations are possible.

Note: The recovery model is a database-level property. You can configure it for each database individually.

Overview of Recovery Models

SQL Server supports three recovery models:

Full Recovery Model
Bulk-Logged Recovery Model
Simple Recovery Model

1. Full Recovery Model

The Full recovery model provides the highest level of data protection and recoverability. In this model:

All transactions are logged in the transaction log.
You can recover the database to any point in time (point-in-time recovery).
This allows for granular recovery of data lost due to user errors or hardware failures.
Requires regular transaction log backups to prevent the log file from growing excessively. If log backups are not performed, the transaction log will fill up, and no new transactions can be processed.

Tip: Use the Full recovery model for critical production databases where minimal data loss is unacceptable.

When to Use Full Recovery Model:

Databases with high transaction volumes.
Databases that require point-in-time recovery.
Production environments where data loss is not an option.

2. Bulk-Logged Recovery Model

The Bulk-Logged recovery model is a hybrid approach that offers a good balance between recoverability and performance for bulk operations. In this model:

Most transactions are logged fully, similar to the Full recovery model.
However, certain bulk operations (like `BULK INSERT`, `SELECT INTO`, `CREATE INDEX`, `ALTER INDEX`) are minimally logged. This means only the pages affected by these operations are logged, not the individual transaction details.
Point-in-time recovery is possible, but it might not be as granular during the time a bulk operation occurred. If a bulk operation is in progress, you can only restore to the end of that operation.
Requires regular transaction log backups.

Important: While the Bulk-Logged recovery model can improve the performance of bulk operations, it also adds complexity to recovery. Ensure you understand the implications before using it.

When to Use Bulk-Logged Recovery Model:

Databases with periodic large bulk data loads.
When the performance benefit of minimally logged bulk operations outweighs the slight reduction in recovery granularity during those operations.

3. Simple Recovery Model

The Simple recovery model is the least protective but also the simplest. In this model:

Transaction log records are automatically de-allocated and overwritten once the transaction is committed and no longer needed for a transaction log backup.
This prevents the transaction log from growing uncontrollably but significantly limits recoverability.
You can only restore the database to the last full backup or differential backup. Point-in-time recovery is not possible.
No transaction log backups are required or can be performed.

Note: If you need to perform transaction log backups or point-in-time recovery, you cannot use the Simple recovery model.

When to Use Simple Recovery Model:

Development or test databases where data loss is not a concern.
Read-only databases.
Small databases that are backed up frequently as full backups.

Changing the Recovery Model

You can change the recovery model of a database using SQL Server Management Studio (SSMS) or Transact-SQL (T-SQL).

Using SSMS:

In Object Explorer, connect to an instance of the SQL Server Database Engine.
Expand the Databases node.
Right-click the database you want to modify, and then select Properties.
In the Database Properties dialog box, select the Options page.
Under the Recovery model list, select the desired recovery model (Full, Bulk-Logged, or Simple).
Click OK.

Using T-SQL:

The syntax to change the recovery model is:

ALTER DATABASE database_name
SET RECOVERY { FULL | BULK_LOGGED | SIMPLE };

For example, to set the recovery model to Full for a database named MyDatabase:

USE master;
GO
ALTER DATABASE MyDatabase
SET RECOVERY FULL;
GO

Important: When switching from Full or Bulk-Logged to Simple recovery, all un-backed-up transaction log entries are lost. Ensure you have taken the necessary transaction log backups before making this change.

Summary Table

Feature	Full Recovery	Bulk-Logged Recovery	Simple Recovery
Point-in-time Recovery	Yes	Yes (with limitations during bulk ops)	No
Transaction Log Backups	Required	Required	Not applicable/possible
Minimal Logging of Bulk Ops	No	Yes	Yes
Log File Growth	Can grow large if not backed up	Can grow large if not backed up	Log space is reused automatically
Data Protection Level	Highest	High	Lowest

Choosing the appropriate recovery model is a fundamental aspect of SQL Server database administration. It directly impacts your ability to recover data in various scenarios and influences the performance of certain operations.