Monday, October 7, 2013

GAB Configuration

Group membership services/Atomic Broadcast (GAB)

GAB provides the following:
  • Group Membership Services - GAB manitains the overall cluster membership by the way of its Group Membership Sevices function. Heartbeats are used to determine if a system is active member, joining or leaving a cluster. GAB determines what the position of a system is in within a cluster.
  • Atomic Broadcast - Cluster configuration and status information is distributed dynamically to all system within the cluster using GAB's Atomic Broadcast feature. Atomic Broadcast ensures all active system receive all messages, for every resource and service group in the cluster. Atomic means that all system receive the update, if one fails then the change is rolled back on all systems.

How To Configure GAB:

·         The /etc/sysconfig/gab file
This file stores the start and stop environment variables for GAB:
o    GAB_START - Defines the startup behavior for the GAB module after a system reboot. Valid values include:
1 - Indicates that GAB is enabled to start up.
0 - Indicates that GAB is disabled to start up.
o    GAB_STOP - Defines the shutdown behavior for the GAB module during a system shutdown. Valid values include:
1 - Indicates that GAB is enabled to shut down.
0 - Indicates that GAB is disabled to shut down.
The installer sets the value of these variables to 1 at the end of VCS configuration.
If you manually configured VCS, make sure you set the values of these environment variables to 1.

The /etc/gabtab file

After you install VCS, the file /etc/gabtab contains a gabconfig(1) command that configures the GAB driver for use.
The file /etc/gabtab contains a line that resembles:
    /sbin/gabconfig -c -nN
The -c option configures the driver for use. The -nN specifies that the cluster is not formed until at least N nodes are ready to form the cluster. Symantec recommends that you set N to be the total number of nodes in the cluster.

 Start GAB

bash-3.00# sh /etc/gabtab
Starting GAB done.

Verifying GAB port membership

Use GAB port membership as a method of determining if a specific component of the SF Oracle RACSF Oracle RAC stack is operating properly and communicating with its peers. The output below shows the common ports in use in a functional SF Oracle RAC environment before fencing is configured. Each line lists a GAB port, such as port a, a generation number determining a startup time, such as gen 4a1c0001, and a membership showing which LLT node IDs are participating, such as membership 01. In the first line of the output below, each node (0 and 1) has membership with the GAB utility that uses port a.
·          To view GAB port membership, type:
/sbin/gabconfig -a
The output resembles this information:
GAB Port Memberships
Port a gen 4a1c0001 membership 01
Port b gen ada40d01 membership 01
Port d gen 40100001 membership 01
Port f gen f1990002 membership 01
Port h gen d8850002 membership 01
Port o gen f1100002 membership 01
Port v gen 1fc60002 membership 01
Port w gen 15ba0002 membership 01
The software configures the ports in the list for these functions:

I/O fencing 
ODM (Oracle Disk Manager) 
CFS (Cluster File System) 
VCS (Veritas Cluster Server: High Availability Daemon) 
VCSMM driver 
CVM (Cluster Volume Manager) 
vxconfigd (module for CVM) 
bash-3.00# /sbin/gabconfig -a
GAB Port Memberships
Port a gen  15a8602 membership ;2
As a final test, reboot both nodes. Once the nodes are up run gabconfig and hastatus to confirm that the cluster is running.
bash-3.00# gabconfig -a
GAB Port Memberships
Port a gen  15a8603 membership ;12
Port h gen  15a8605 membership ;12


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