IBM PowerHA SystemMirror (formerly IBM PowerHA and HACMP) is IBM's solution for high-availability clusters on the AIX Unix and Linux for IBM System p platforms and stands for High Availability Cluster Multiprocessing. IBM's HACMP product was first shipped in 1991 and is now in its 20th release - PowerHA SystemMirror for AIX 7.1.

PowerHA can run on up to 32 computers or nodes, each of which is either actively running an application (active) or waiting to take over when another node fails (passive). Data on file systems can be shared between systems in the cluster.

PowerHA relies heavily on IBM's Reliable Scalable Cluster Technology (RSCT). PowerHA is an RSCT aware client. RSCT is distributed with AIX. RSCT includes a daemon called group services that coordinates the response to events of interest to the cluster (for example, an interface or a node fails, or an administrator makes a change to the cluster configuration). Up until PowerHA V6.1, RSCT also monitored cluster nodes, networks and network adapters for failures using the topology services daemon (topsvcs). In the current release (V7.1), RSCT provides coordinate response between nodes, but monitoring and communication are provided by the Cluster Aware AIX (CAA) infrastructure.

The 7.1 release of PowerHA relies heavily on CAA, a clustering infrastructure built into the operating system and exploited by RSCT and PowerHA. CAA provides the monitoring and communication infrastructure for PowerHA and other clustering solutions on AIX, as well as cluster-wide event notification using the Autonomic Health Advisor File System (AHAFS) and cluster-aware AIX commands with clcmd. CAA replaces the function provided by Topology Services (topsvcs) in RSCT in previous releases of PowerHA/HACMP [1].