Shared external disk devices

Each node has access to one or more shared external disk devices. A shared external disk device is a disk physically connected to multiple nodes.

The shared disk stores mission-critical data, typically mirrored or RAID-configured for data redundancy. A node in a PowerHA® SystemMirror® cluster must also have internal disks that store the operating system and application binaries, but these disks are not shared. Depending on the type of disk used, the PowerHA SystemMirror software supports the following types of access to shared external disk devices - nonconcurrent access and concurrent access.

• In nonconcurrent access environments, only one connection is active at any given time, and the node with the active connection owns the disk. When a node fails, the node that currently owns the disk leaves the cluster, disk takeover occurs and a surviving node assumes ownership of the shared disk. This typical cluster configuration is used by most applications.
• In concurrent access environments, the shared disks are actively connected to more than one node simultaneously. Therefore, when a node fails, disk takeover is not required. This type of access is used only by applications that can manage and coordinate simultaneous access to shared data from multiple nodes.

Note that in such environments, either all nodes defined in the cluster can be part of the concurrent access, or only a subset of cluster nodes can have access to shared disks. In the second case, you configure resource groups only on those nodes that have shared disk access. The differences between these methods are explained more fully in the section Shared external disk access.