posted Jan 28, 2012, 8:31 AM by Sachchida Ojha   [ updated Jan 28, 2012, 8:50 AM ]
When you configure a RAC system, one of the tasks you have to do at the OS level is assign each machine a "VIP". Immediately after startup the cluster service software inserts this code layer above the real IP address, and the machines switch to communicating through the VIP's.
SCAN Address

The big difference between real and virtual IP addresses is that the real IP address is logically bound to a specific piece of hardware (MAC address on the network card); whereas the VIP address is controlled by software and its association with a particular piece of hardware can be changed dynamically.

If a database machine were to fail when a client program was attached to it through the real IP, the client program would wait a long time (tens to hundreds of seconds) for a response from the server before deciding that the server machine had failed. Because Oracle is using VIPs, when a machine fails, one of the other instance will detect the failure very rapidly and take on the virtual IP of the failed machine to handle the current requests. This reconfigures the system to stop future connections going to a failed VIP (until the machine and instance are live again). This means that clients will not experience a long wait if the instance they are addressing fails.

SCAN (Single Client Access Name): Scan is the another aid to minimize waits caused by network problems. Oracle 11.2 introduces SCAN as part of Oracle Grid Infrastructure.

New SRVCTL Command

What it does

srvctl config scan

Shows the current SCAN configuration

srvctl config scan_listener

Shows the existence and port numbers for the SCAN listeners

srvctl add scan -n cluster01-scan

Adds new SCAN information for a cluster

srvctl remove scan -f

Removes SCAN information

srvctl add scan_listener

Adds a new SCAN listener for a cluster on the default port of 1521

srvctl add scan_listener -p 65001 ## non default port number ##

Adds a new SCAN listener on a different port

srvctl remove scan_listener

Removes the SCAN listener

srvctl modify scan -n cluster_scan

Modifies SCAN information (used when changing SCAN to DNS after initially using /etc/hosts)

srvctl modify scan_listener -u

Modifies the SCAN listener information to match the new SCAN VIP information from the modify scan command

Sachchida Ojha,
Jan 28, 2012, 8:38 AM