Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 6 - Install, Configure and Database Creation

What is VSD?

The VSD is a logical volume that can be accessed both by the node to which it belongs and also by other nodes in the system. The VSD nodes that share data may reside in a single system partition. VSD software allows applications running on different nodes to have access to raw logical volumes as if they were local. I/O requests to VSD(s) are routed by the VSD device driver, which is loaded as a kernel extension on each node, thus making raw logical volumes accessible to other nodes in the system.

Depending on the VSD function, a node can be:

VSD Server that has the local attached disks. It is able to complete I/O requests from VSD clients by using a communication network inside the system.

A VSD Client that is a node requesting access to VSD(s).

There is another variation for VSD, which is known as Concurrent VSD. The Concurrent Virtual Shared Disk feature of PSSP allows multiple VSD servers to simultaneously access logical volumes inside a volume group using the Concurrent Logical Volume Manager (CLVM) component supplied by AIX. I/O requests from nodes (that do not have locally attached disks) are spread across VSD servers, thus improving raw logical volume access.

When we use the Concurrent Virtual Shared Disk, recovery from node failure is much faster because the failed node is marked as unavailable to all other nodes. Its access to the physical disk is fenced, while the other nodes can continue to access the disks. Virtual Shared Disks provide access to non-local disks. They behave like raw logical volumes. They can be configured using SMIT or commands. VSD(s) are accessed through the SP Switch so the network overhead is very low.