- a node manager (o2nm) to keep track of the nodes in the cluster,
- a heartbeat agent (o2hb) to detect live nodes
- a network agent (o2net) for intra-cluster node communication
- a distributed lock manager (o2dlm) to keep track of lock resources
- All these components are in-kernel.
- It also includes an in-memory file system, dlmfs, to allow userspace to access the in-kernel dlm
- /etc/ocfs2/cluster.conf, /etc/sysconfig/o2cb, /sys/kernel/config/cluster
|02net||The o2net process creates TCP/IP intra-cluster node communication channels on port 7777 and sends regular keep-alive packages to each node in the cluster to validate if the nodes are alive. The intra-cluster node communication uses the network with the Cluster Heartbeat role. By default, this is the Server Management network. You can however create a separate network for this function. See Section 5.2, “Network Usage” for information about the Cluster Heartbeat role. Make sure the firewall on each Oracle VM Server in the cluster allows network traffic on the heartbeat network. By default, the firewall is disabled on Oracle VM Servers after installation.|
|o2hb-diskid||The server pool cluster also employs a disk heartbeat check. The o2hb process is responsible for the global disk heartbeat component of cluster. The heartbeat feature uses a file in the hidden region of the server pool file system. Each pool member writes to its own block of this region every two seconds, indicating it is alive. It also reads the region to maintain a map of live nodes. If a server pool member's block is no longer updated, the Oracle VM Server is considered dead. If an Oracle VM Server dies, the Oracle VM Server is fenced. Fencing forcefully removes dead members from the server pool to make sure active pool members are not obstructed from accessing the fenced Oracle VM Server's resources.|
|o2cb||The o2cb service is central to cluster operations. When an Oracle VM Server boots, the o2cb service starts automatically. This service must be up for the mount of shared repositories to succeed.|
|ocfs2||The ocfs2 service is responsible for the file system operations. This service also starts automatically.|
|ocfs2_dlm and ocfs2_dlmfs||The DLM modules (ocfs2_dlm, ocfs2_dlmfs) and processes (user_dlm, dlm_thread, dlm_wq, dlm_reco_thread, and so on) are part of the Distributed Lock Manager.
OCFS2 uses a DLM to track and manage locks on resources across the cluster. It is called distributed because each Oracle VM Server in the cluster only maintains lock information for the resources it is interested in. If an Oracle VM Server dies while holding locks for resources in the cluster, for example, a lock on a virtual machine, the remaining Oracle VM Servers in the server pool gather information to reconstruct the lock state maintained by the dead Oracle VM Server.
Here is more about ocfs2 and o2cb http://docs.oracle.com/cd/E37670_01/E37355/html/ol_ocfs2.html