Shared external disk devices

Each node must have 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 an HACMP™ 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, HACMP™ supports two types of access to shared external disk devices: non-concurrent and concurrent access.

• In non-concurrent access environments, only one connection is active at any time, and the node with the active connection owns the disk. When a node fails, disk takeover occurs when the node that currently owns the disk leaves the cluster and a surviving node assumes ownership of the shared disk.
• 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.