SQL Server is a powerful database management system that is widely used in various industries. One of the key features of SQL Server is its ability to provide high availability through failover clustering. However, there are certain behaviors and considerations that database administrators need to be aware of when it comes to failover scenarios.

The Issue with TEMPDB

In SQL Server, TEMPDB is a system database that is used to store temporary objects such as temporary tables, table variables, and cursors. It plays a crucial role in query processing and can significantly impact the performance of your SQL Server instance.

When using local disks for TEMPDB on a failover cluster, there is a caveat. The local disk becomes a non-clustered resource, and the cluster service has no way of knowing the health state of the local disk. This means that if the TEMPDB local disk fails, the SQL Server instance does not shut down, making it practically inoperable.

This behavior can be surprising and concerning for database administrators who expect the SQL Server instance to initiate a failover in such scenarios. However, Microsoft has confirmed that this is intended behavior, as long as the other databases are still available and query-able.

The Expected Behavior

In a failover cluster configuration, it is reasonable to expect that if TEMPDB becomes unusable due to a disk error, the SQL Server instance should initiate a failover to ensure high availability. This would allow the instance to come up on another node where there is a working TEMPDB.

However, in the current behavior, the SQL Server instance continues to run even without a working TEMPDB, and the cluster service does not initiate a failover. This can have implications for the overall functionality and reliability of the application that relies on the SQL Server instance.

Considerations for High Availability

For administrators who want to ensure high availability in their SQL Server environment, there are a few considerations to keep in mind:

• Place TEMPDB on a cluster storage resource: If possible, it is recommended to place TEMPDB on a cluster storage resource rather than local disks. This helps to minimize the risk of failure and ensures that the cluster service can monitor the health of the storage.
• Implement redundancy on local storage: If TEMPDB needs to be placed on local storage, it is important to implement redundancy at the hardware level, such as RAID1 or RAID10. This can help mitigate the risk of disk failure and improve the overall reliability of the SQL Server instance.
• Ensure network access is highly redundant: If TEMPDB is placed on CIFS/SMB, it is crucial to ensure that network access is highly redundant. This helps to prevent any network-related issues from impacting the availability of TEMPDB.
• Consider implications for Always On: Similar issues may arise with Always On implementations. It is important to be aware that a failing TEMPDB will not trigger a failover to any secondary replica in an availability group.

Alternative Approaches

For experienced database administrators who want to ensure failover in case of TEMPDB failure, there are alternative approaches that can be considered:

• Creating an external PowerShell script: This script can monitor the health of the disk by watching for specific error messages, such as the “Msg 9001” error. If the script detects a failure, it can initiate a failover of the SQL Server resource.
• Using database level health check in Always On: If you are using SQL Server Always On, you can create a health probe by using database level health check and the transaction log of user databases on the same local disk as TEMPDB. This can help detect any issues with TEMPDB and trigger a failover if necessary.

By implementing these alternative approaches, database administrators can have more control over the failover behavior of their SQL Server instances and ensure the highest level of availability for their applications.

In conclusion, understanding the failover behavior of SQL Server, especially in relation to TEMPDB, is crucial for ensuring high availability and reliability. By considering the recommendations and alternative approaches mentioned in this article, database administrators can better manage and mitigate the risks associated with TEMPDB failure.

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