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.