Monday, 18 July 2011

Heartbeating in HACMP

The primary task of HACMP is to recognize and respond to failures. HACMP uses heartbeating to monitor the activity of its network interfaces, devices and IP labels.

Heartbeating connections between cluster nodes are necessary because they enable HACMP to recognize the difference between a network failure and a node failure. For instance, if connectivity on the HACMP network (this network's IP labels are used in a resource group) is lost, and you have another TCP/IP based network and a non-IP network configured between the nodes, HACMP recognizes the failure of its cluster network and takes recovery actions that prevent the cluster from becoming partitioned.

To avoid cluster partitioning, we highly recommend configuring redundant networks in the HACMP cluster and using both IP and non-IP networks. Out of these networks, some networks will be used for heartbeating purposes.

In general, heartbeats in HACMP can be sent over:
1)TCP/IP networks.
2)Serial (non-IP) networks (RS232, TMSCSI, TMSSA and disk heartbeating).
The Topology Services component of RSCT carries out the heartbeating function in HACMP.

Topology services and heartbeat rings
HACMP uses the Topology Services component of RSCT for monitoring networks and network interfaces. Topology Services organizes all the interfaces in the topology into different heartbeat rings. The current version of RSCT Topology services has a limit of 48 heartbeat rings, which is usually sufficient to monitor networks and network interfaces.

Heartbeat rings are dynamically created and used internally by RSCT. They do not have a direct, one-to-one correlation to HACMP networks or number of network interfaces. The algorithm for allocating interfaces and networks to heartbeat rings is complex, but generally follows these rules:

!)In an HACMP network, there is one heartbeat ring to monitor the service interfaces, and one for each set of non-service interfaces that are on the same subnet. The number of non-service heartbeat rings is determined by the number of non-service interfaces in the node with the largest number of interfaces.
2)The number of heartbeat rings is approximately equal to the largest number of interfaces found on any one node in the cluster.

Note that during cluster verification, HACMP calls the RSCT verification API. This API performs a series of verifications, including a check for the heartbeat ring calculations, and issues an error if the limit is exceeded.

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