CVM is designed with a "master and slave" architecture. One node in the cluster acts as the configuration master for logical volume management, and all other nodes are slaves. Any node can take over as master if the existing master fails. The CVM master exists on a per-cluster basis and uses GAB and LLT to transport its configuration data.

Just as with VxVM, the Volume Manager configuration daemon, vxconfigd, maintains the configuration of logical volumes. This daemon handles changes to the volumes by updating the operating system at the kernel level. For example, if a mirror of a volume fails, the mirror detaches from the volume and vxconfigd determines the proper course of action, updates the new volume layout, and informs the kernel of a new volume layout. CVM extends this behavior across multiple nodes and propagates volume changes to the master vxconfigd.

The vxconfigd process on the master pushes these changes out to slave vxconfigd processes, each of which updates the local kernel. The kernel module for CVM is kmsg.

See Figure: Low-level communication.

CVM does not impose any write locking between nodes. Each node is free to update any area of the storage. All data integrity is the responsibility of the upper application. From an application perspective, standalone systems access logical volumes in the same way as CVM systems.

By default, CVM imposes a "Uniform Shared Storage" model. All nodes must connect to the same disk sets for a given disk group. Any node unable to detect the entire set of physical disks for a given disk group cannot import the group. If a node loses contact with a specific disk, CVM excludes the node from participating in the use of that disk.

Set the storage_connectivity tunable to asymmetric to enable a cluster node to join even if the node does not have access to all of the shared storage. Similarly, a node can import a shared disk group even if there is a local failure to the storage.

For detailed information, see the Symantec Storage Foundation Cluster File System High Availability Administrator's Guide.