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In recent years, tightly-coupled cluster systems have become increasingly popular in the realm of enterprise-scale mission-critical data processing. The primary advantage of clusters is protection against hardware failure. Should the primary node fail or otherwise become 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. Businesses can derive enhanced value from their investment in cluster systems by performing such operations on lightly loaded nodes in the cluster rather than 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.
The cluster functionality of VxVM works together with the cluster monitor daemon that is provided by VCS or by the host operating system. When configured correctly, 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 disk groups and volumes. When a node leaves a cluster, it no longer has access to these shared objects. A node joins a cluster when the cluster monitor is started on that node.
Example of a 4-node cluster shows 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).
Example of a 4-node cluster
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Node 0 is configured as the master node and nodes 1, 2 and 3 are configured as slave nodes. 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 SCSI or Fibre Channel.
In this example, each node has two independent paths to the disks, which are configured in one or more cluster-shareable disk groups. Multiple paths provide resilience against failure of one of the paths, but this is not a requirement for cluster configuration. Disks may also be connected by single paths.
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.
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.
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 determines that the first node to join a cluster performs the function of master node. If the master node leaves a cluster, one of the slave nodes is chosen to be the new master.