The Geek Diary

Configuring resource groups In most cases, having the cluster run individual resources is not an ideal situation. Mounting a file system with web content on one node, starting an httpd instance on a second, and assigning a floating IP address to a third node does not create a service that consumers can use. Resources can be combined into resource groups that will force all resources in a group to start and stop at the same time, on the same node, creating clustered services for consumers. … [Read more...] about How to Create and Configure Resource Groups in a Pacemaker Cluster

Resources Clustered services consist of one or more resources. A resource can be an IP address, a file system, or a service like httpd, among others. All the bits required to provide a service to consumers are a resource. With the Red Hat High Availability Add-on, all resources are monitored independently from each other. In order to accomplish this, resources are controlled by resource agents. Pacemaker can manage different kinds of resource agents: AgentPurpose LSBLinux Standard … [Read more...] about How to Create and Configure Resources in a Pacemaker Cluster

Creating a fence device A system administrator can create a fence device in the cluster with the pcs stonith create command: # pcs stonith create name fencing_agent parameters The command requires additional arguments, such as the fencing agent and the fencing parameters required for the fencing agent. There are generic properties for all the fencing agents shipped with Red Hat Enterprise Linux High Availability Add-on: stonith-timeout This setting defines the time to wait for fencing … [Read more...] about Configuring Cluster Fencing Agents in a Pacemaker Cluster

Overview of fence device configuration Fencing is a requirement for every operational cluster. The first step when setting up fencing for the cluster is the setup of the hardware or software device that does the actual fencing. The Red Hat Enterprise Linux High Availability Add-on provides a variety of fencing agents for use with different fence devices. The pcs stonith list provides a list of all installed fencing agents: # pcs stonith list fence_apc - Fence agent for APC over … [Read more...] about Setting Up Fencing Devices in a Pacemaker Cluster

What is fencing? The Red Hat High Availability Add-on uses fencing to ensure data integrity in the cluster. Fencing is often accomplished by powering the node off since a dead node is clearly not able to do anything. In other cases, a combination of operations will be used to cut the node off from the network (to stop new work from arriving) or from storage (to stop the node from writing to shared storage). Fencing is a necessary step in service and resource recovery in a cluster. The Red Hat … [Read more...] about What is fencing and What are different methods of fencing in a pacemaker cluster

Set quorum calculation options The votequorum component allows a cluster administrator to build clusters with switches that alter the way quorum gets calculated. When building a new cluster with the pcs cluster setup command, the following switches may be added to change the behavior of quorum handling in the cluster: --wait_for_all Wait for all cluster members to be online before starting to calculate quorum. This setting effectively prevents a fence race when the cluster is starting up. … [Read more...] about Managing Quorum Calculations in a Pacemaker Cluster

What is quorum? In order for a cluster to work as expected, the nodes must be in agreement on certain facts, such as which machines are currently cluster members, where services are running, and which machines are using which resources. The method in which this is implemented in the Red Hat High Availability Add-on is by the use of a majority voting scheme. Every cluster node casts one vote if it successfully joins the corosync network communication and is able to communicate with the other … [Read more...] about What is quorum in a pacemaker cluster (Understanding Quorum Operations)

There are times when an administrator needs to temporarily suspend resource hosting of a cluster node without disrupting regular cluster operation. This can happen, for example, when a critical security update needs to be applied for the hosted resource. The update can be applied after putting node by node into standby mode, resulting in reduced downtime. A different use case is to test resource migration. When a node is put in standby mode, it does not get any resources assigned. Resources that … [Read more...] about Prohibiting a cluster node from hosting resources in pacemaker cluster (putting a node into standby mode)

The Red Hat High Availability Add-on provides different ways for a system administrator to control membership of cluster nodes: Starting and stopping the cluster services control whether a cluster node is participating in the cluster at runtime. Enabling and disabling the cluster services control whether a cluster node automatically starts the cluster services and joins the cluster when it boots. Adding and removing a cluster node from the cluster permanently changes whether the node is a … [Read more...] about Managing Cluster Membership in pacemaker cluster (Adding and removing a cluster node)