Oracle VM Server : How do Server Pool Clusters Work?

http://docs.oracle.com/cd/E50245_01/E50249/html/vmcon-svrpool-cluster.html

Oracle VM works in concert with Oracle OCFS2 to provide shared access to server pool resources residing in an OCFS2 file system. This shared access feature is crucial in the implementation of high availability (HA) for virtual machines running on x86 Oracle VM Servers that belong to a server pool with clustering enabled.
OCFS2 is a cluster file system developed by Oracle for Linux, which allows multiple nodes (Oracle VM Servers) to access the same disk at the same time. OCFS2, which provides both performance and HA, is used in many applications that are cluster-aware or that have a need for shared file system facilities. With Oracle VM, OCFS2 ensures that Oracle VM Servers belonging to the same server pool access and modify resources in the shared repositories in a controlled manner.

The OCFS2 software includes the core file system, which offers the standard file system interfaces and behavioral semantics and also includes a component which supports the shared disk cluster feature. The shared disk component resides mostly in the kernel and is referred to as the O2CB cluster stack. It includes:

• A disk heartbeat to detect live servers.
• A network heartbeat for communication between the nodes.
• A Distributed Lock Manager (DLM) which allows shared disk resources to be locked and released by the servers in the cluster.

Oracle VM decouples storage repositories and clusters so that if a storage repository is taken off-line, the cluster is still available. A loss of one heartbeat device does not force an Oracle VM Server to self fence.
When you create a server pool, you have a choice to activate the cluster function which offers these benefits:

• Shared access to the resources in the repositories accessible by all Oracle VM Servers in the cluster.
• Protection of virtual machines in the event of a failure of any Oracle VM Server in the server pool.

You can choose to configure the server pool cluster and enable HA in a server pool, when you create or edit a server pool within Oracle VM Manager.

As Oracle VM Servers are added to a newly created server pool, Oracle VM:

1. Selects a Master Oracle VM Server.
2. Configures the Virtual IP address selected during pool creation as a virtual network interface on top of the management interface for the Master Oracle VM Server.
3. Creates the cluster configuration file and the cluster time-out file.
4. Pushes the configuration files to all Oracle VM Servers in the server pool.
5. Starts the cluster.

On each Oracle VM Server in the cluster, the cluster configuration file is located at /etc/ocfs2/cluster.conf, and the cluster time-out file is located at /etc/sysconfig/o2cb. Cluster timeout can be configured during server pool creation.


 With this information in mind, the description provided in Section 6.8.1, “Clustering for x86 Server Pools” largely applies equally to clustering on SPARC, although the implementation does not use OCFS2.