How Cluster Components Affect Performance

VCS and its agents run on the same systems as the applications. Therefore, VCS attempts to minimize its impact on overall system performance. The three main components of clustering that have an impact on performance include the kernel; specifically, GAB and LLT, the VCS engine (HAD), and the VCS agents. For details on attributes or commands mentioned in the following sections, see the chapter on administering VCS from the command line and the appendix on VCS attributes.

Kernel Components (GAB and LLT)

Typically, overhead of VCS kernel components is minimal. Kernel components provide heartbeat and atomic information exchange among cluster systems. By default, each system in the cluster sends two small heartbeat packets per second to other systems in the cluster. Heartbeat packets are sent over all network links configured in the /etc/llttab configuration file. System-to-system communication is load-balanced across all private network links. If a link fails, VCS continues to use all remaining links. Typically, network links are private and do not increase traffic on the public network or LAN. You can configure a public network (LAN) link as low-priority, which by default generates a small (approximately 64-byte) broadcast packet per second from each system, and which will carry data only when all private network links have failed.