CentOS Cluster Configuration

cluster.conf configuration file

Configuration file for:

• cman - Cluster configuration
• fence - Fence configuration for disabling nodes with errors
• dlm - Distributed Lock Manager Configuration. Rules for access to shared resources
• gfs - Global file System configuration. Shared file systems among nodes.
• rgmanager - Resource Group manager configuration. Fx. apache service setup on cluster.

See RedHAT 5 cluster scheme

rgmanager - resource config

Resource Group manager is a High Availability service. Rgmanager can start and stop services on nodes. If a service is failing on one node it will be started on another node. Rgmanager monitor the services and make sure they are actually runnning.

Service Groups

A Service Group is a group of nodes on which a specified service can be started or stopped by rgmanager. Not all nodes in a cluster need to be member of a Service Group. There can be many Service Groups in a cluster.If a service fails, a script is called to automatically restart the service. If a node fails, the service may be relocated to a different node in the service group.

What is a Cluster-service

A Cluster-service is a resource that are shared among nodes. For example a apache WEB-service. This service can be run in two different ways.

active-passive Cluster-service

An active-passive Cluster-service is a service running on one node at a time. If the node running the service fails the service is started on another node in the Service Group.

active-passive example

Two front-end-nodes have the responsibility of delivering a high-availability WEB-service. In the image below there are three services

1. Filesystem: The filesystem is a ext3 filesystem which can only be mounted on one node at a time. In Picture 1 below the left node has mounted the shared filesystem on the SCSI-raid controllers. The other servers has not mounted the shared filesystem.
2. Service: The Apache WEB-server service is running on the left node, and using the resources from the shared SCSI filesystem delivering WEB-content to the Internet. The Apache service on the other nodes are stopped.
3. IP address: The left server is answering request for the shared IP address 80.1.2.3. The other nodes ignores 80.1.2.3.

Picture 1: Active-passive setup WEB-server

files and programs

• /usr/share/cluster - here lives the rgmanager scripts
• /etc/cluster/cluster.conf - rgmanager configuration
• clustat - See cluster and service status clustat -s SERVICE_NAME -l
• RedHAT rgmanager FAQ