||Oracle Database Tips by Donald Burleson
Oracle10g Grid Computing
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.
The above text is
an excerpt from:
Oracle 10g Grid & Real Application
Computing with RAC
by Mike Ault, Madhu Tumma