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Tightly coupled cluster systems have become increasingly popular in enterprise-scale mission-critical data processing. The primary advantage of clusters is protection against hardware failure. If the primary node fails or otherwise becomes unavailable, applications can continue to run by transferring their execution to standby nodes in the cluster. This ability to provide continuous availability of service by switching to redundant hardware is commonly termed failover.
Another major advantage of clustered systems is their ability to reduce contention for system resources caused by activities such as backup, decision support and report generation. Enhanced value can be derived from cluster systems by performing such operations on lightly loaded nodes in the cluster instead of on the heavily loaded nodes that answer requests for service. This ability to perform some operations on the lightly loaded nodes is commonly termed load balancing.
To implement cluster functionality, VxVM works together with the cluster monitor daemon that is provided by the host operating system or by VCS. The cluster monitor informs VxVM of changes in cluster membership. Each node starts up independently and has its own cluster monitor plus its own copies of the operating system and VxVM with support for cluster functionality. When a node joins a cluster, it gains access to shared disks. When a node leaves a cluster, it no longer has access to shared disks. A node joins a cluster when the cluster monitor is started on that node.
Note
The cluster functionality of VxVM is supported only when used in conjuction with a cluster monitor that has been configured correctly to work with VxVM.
Example of a 4-node cluster illustrates a simple cluster arrangement consisting of four nodes with similar or identical hardware characteristics (CPUs, RAM and host adapters), and configured with identical software (including the operating system).
The nodes are fully connected by a private network and they are also separately connected to shared external storage (either disk arrays or JBODs: just a bunch of disks) via Fibre Channel. Each node has two independent paths to these disks, which are configured in one or more cluster-shareable disk groups.
The private network allows the nodes to share information about system resources and about each other's state. Using the private network, any node can recognize which other nodes are currently active, which are joining or leaving the cluster, and which have failed. The private network requires at least two communication channels to provide redundancy against one of the channels failing. If only one channel were used, its failure would be indistinguishable from node failure—a condition known as network partitioning.
Example of a 4-node cluster
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To the cluster monitor, all nodes are the same. VxVM objects configured within shared disk groups can potentially be accessed by all nodes that join the cluster. However, the cluster functionality of VxVM requires that one node act as the master node; all other nodes in the cluster are slave nodes. Any node is capable of being the master node, and it is responsible for coordinating certain VxVM activities.
Note
You must run commands that configure or reconfigure VxVM objects on the master node. Tasks that must be initiated from the master node include setting up shared disk groups, creating and reconfiguring volumes, and performing snapshot operations.
VxVM designates the first node to join a cluster performs the function of the master node. If the master node leaves the cluster, one of the slave nodes is chosen to be the new master.
In Example of a 4-node cluster node 0 is the master node and nodes 1, 2 and 3 are slave nodes.