VCS Overview

The VERITAS Cluster Server provides the communication, configuration, and membership services required to create a cluster. VCS is the first component installed and configured to set up a cluster file system.

GAB and LLT are VCS-specific protocols implemented directly on an Ethernet data link or on a Fibre Channel fabric. Both GAB and LLT run over redundant data links that connect all the servers in a cluster. VCS requires redundant cluster communication links to minimize the possibility of cluster failure due to the failure of a single communication link.

VCS Messaging---GAB

GAB provides membership and messaging service, both for the cluster as a whole and for groups of applications running it. The GAB membership service provides orderly startup and shutdown of a cluster.

The file /etc/gabtab is used to configure GAB. Configuration is done with the gabconfig command. For example, the –n option of the command specifies the number of nodes in the cluster. GAB is configured automatically when you run the VCS installation script, but you may have to reconfigure GAB when you add a node to a cluster. See the gabconfig(1m) manual page for additional information.

VCS Communication---LLT

LLT provides kernel-to-kernel communications and monitors network communications. The LLT files /etc/llthosts and /etc/llttab can be configured to set system IDs within a cluster, set cluster IDs for multiple clusters, and tune network parameters such as heartbeat frequency. LLT is implemented so that events such as state changes are reflected quickly, which in turn enables fast responses.

As with GAB, LLT is configured automatically when you run the VCS installation script. The file /etc/llttab contains information derived from what you input during installation. You may also have to reconfigure LLT when you add a node to a cluster. See the llttab(4) manual page for details on how you can modify the LLT configuration.