Veritas Storage Foundation 5.1 SP1 for Oracle RAC Administrator"s Guide (5900-1512, April 2011)

Cluster Volume Manager (CVM)

CVM is an extension of Veritas Volume Manager, the industry-standard storage

virtualization platform. CVM extends the concepts of VxVM across multiple nodes.

Each node recognizes the same logical volume layout, and more importantly, the

same state of all volume resources.

CVM supports performance-enhancing capabilities, such as striping, mirroring,

and mirror break-off (snapshot) for off-host backup. You can 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 user interaction.

For detailed information, see the Veritas Volume Manager Administrator's Guide.

CVM architecture

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.

Note: You must perform operator-initiated changes on the master node.

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.

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 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

Overview of Veritas Storage Foundation for Oracle RAC

Component products and processes of SF Oracle RAC