Better Support for Windows Server Failover Clusters with Asymmetric Storage

Support for asymmetric storage configurations with Windows Server Failover Clusters (WSFC) was added to Windows Server 2008 with hotfix KB976097 and is available with RTM versions of Windows Server 2012, 2012R2 and 2016.

SQL Server 2014 AlwaysON availability groups make extensive use of asymmetric storage configurations. Indeed Microsoft best practice mandates that if availability replicas of a database are established between a SQL Server Failover Cluster Instance (FCI) and a stand-alone SQL instance, or between two SQL Server FCIs, then all the hosts should be contained in a single Windows Server Failover Cluster. Moreover, the file path to the database replicas should be the same on both the primary and replica copies of the database.

Following these guidelines results in one of the following scenarios:
- File systems at the local site which are clustered on the FCI will have the same drive letters assigned as non-clustered file systems on the stand-alone SQL instance in the remote site. Since all hosts are in the same WSFC, the Spectrum Protect Backup/Archive client confuses the file-system on the stand-alone server with the clustered file system and does not allow it to be backed up using the namespace of the server hosting the stand-alone SQL instance.
- File systems in one site which are clustered on the local FCI will have the same drive letters assigned as the file systems in the remote site which are also clustered on the remote FCI. The Spectrum Protect Backup/Archive client expects the drive letters associated with filesystems to be unique within a single WSFC and cannot make the distinction between two file systems with the same drive letter assigned in the same WSFC.

The cluster configurations that the Backup/Archive client must be changed to accommodate are detailed in the Microsoft document called AlwaysOn Architecture Guide: Building a High Availability and Disaster Recovery Solution by Using Failover Cluster Instances and Availability Groups

In general, the Backup/Archive client must be able to cope with both the following scenarios (perhaps simultaneously)
- The set of drive letters assigned to the local disks / file systems of the hosts within a single WSFC and the set of drive letters assigned to the clustered disks / file systems are not mutually exclusive; and
- Multiple different clustered file systems within the same WSFC are assigned the same drive letter.