Cluster Volume Manager (CVM)

CVM is an extension of VERITAS Volume Manager (VxVM), the industry standard storage virtualization platform. CVM extends the concepts of VxVM across multiple nodes. Each node recognizes the same logical volume layout and state of all volume resources.

Use standard VxVM commands from one node in the cluster to manage all storage. All other nodes immediately recognize any changes in disk group and volume configuration with no interaction. CVM supports performance-enhancing capabilities such as striping, mirroring, and mirror break-off (snapshot) for off-host backup.

CVM Architecture

CVM is designed with a "master/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.

Since CVM is an extension of VxVM, it operates in a similar fashion. 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 changes to a volume 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.

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.

CVM imposes a "Uniform Shared Storage" model. All systems must be connected to the same disk sets for a given disk group. Any system unable to see 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.