# Failover installation guidelines and restrictions

Hardware requirements | Installation guidelines and restrictions | Preparing the controllers

Important If the following guidelines and restrictions are not met, failover is disabled for the controller pair, and an error explaining the problem is logged in the system event log.

To achieve fault tolerance with ServeRAID, you must adhere to the following guidelines and restrictions:

- You must configure the controllers as an active-passive pair, where all logical drives are defined on only the active controller.

  Note: If both controllers have logical drives configured, the controllers will not be configured as a failover pair.

- The stripe-unit size and level of BIOS and firmware code must be the same on both controllers.
- A failover pair must not contain any physical drives in the ready state. All physical drives must be configured in arrays.
- Hot-spare drives are not supported in a failover environment. The exception is the built-in hot-spare that is part of a RAID level-5E logical drive.
- RAID level-x0 is not supported in a failover environment.
- All logical drives must have unique shared merge-group numbers.
- You must set the write-cache mode of all logical drives on the controller to write through.

  Note: If your ServeRAID controller has a battery-backup cache, it is disabled when you create a failover pair. The write-cache mode is automatically changed from write back to write through.

- A failover pair and a ServeRAID adapter defined as a Microsoft Cluster Service (MSCS) resource cannot coexist in the same server.
- When you use multiple failover pairs in a system, each set of controllers must have unique controller and partner names.
- You cannot hot add a controller and then configure it as part of a failover pair.
- All on-board ServeRAID controllers must be enabled, even if the controller is not used.
- A logical drive cannot failover if it is any of the following:
  - A RAID level-5 or level-5E logical drive in a critical state
  - A RAID level-5 or level-5E logical drive that is undergoing a rebuild operation
  - A RAID level-5E logical drive that is undergoing a compression or decompression operation
  - A logical drive undergoing a logical-drive migration

## More information

- Understanding the hardware requirements
- Preparing the controllers for failover
- Connecting the enclosures
- Configuring the arrays and logical drives
- Completing the failover environment installation
- Failing from active to passive controller (action)

Glossary | Contents

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