Pre-existing Network Partitions

A pre-existing network partition refers to failures in communication channels that occur while the systems are down. Regardless of whether the cause is scheduled maintenance or system failure, VCS cannot respond to failures when systems are down. This leaves VCS without I/O fencing vulnerable to network partitioning when the systems are booted. VCS seeding is designed to help prevent this situation in clusters without I/O fencing

VCS Seeding

To protect your cluster from a pre-existing network partition, VCS employs the concept of a seed. Systems can be seeded automatically or manually. Note that only systems that have been seeded can run VCS.

By default, when a system comes up, it is not seeded. When the last system in a cluster is booted, the cluster seeds and starts VCS on all systems. Systems can then be brought down and restarted in any combination. Seeding is automatic as long as at least one instance of VCS is running in the cluster.

Automatic seeding occurs in one of two ways:

• When an unseeded system communicates with a seeded system.
• When all systems in the cluster are unseeded and able to communicate with each other.

VCS requires that you declare the number of systems that will participate in the cluster.

Seeding control is established via the /etc/gabtab file. GAB is started with the command /sbin/gabconfig -c -n X. The variable X represents number of nodes in the cluster.

To start a cluster with less than all nodes, first verify the nodes not to be in the cluster are down, then start GAB using the command /sbin/gabconfig -c -x. This manually seeds the cluster and allows VCS to start on all connected systems.

During initial startup, VCS autodisables a service group until all resources are probed for the group on all systems in the SystemList that have GAB running. This protects against a situation where enough systems are running LLT and GAB to seed the cluster, but not all systems have HAD running.