How To Stop LLT And GAB In VCS

In this article, we are going to understand how to stop LLT and GAB in VCS properly and start them and verify cluster status. This may be needed as part of a troubleshooting effort while investigating issues in the cluster. So let us understand briefly what is LLT and GAB and what is their role in VCS.

Table of Contents

What is LLT

LLT is basically a transport layer protocol used to transmit heartbeats between cluster nodes. It stands for low latency transport. LLT helps GAB to determine the state of the node in the cluster. It helps in inter-system communication traffic equally among all nodes. Including high and low priority links total of 8 LLT links can be created.

LLT status of the cluster can be verified using the below command. The output is truncated.

[root@linsrv01]# lltstat -nvv |more
LLT node information:
    Node                 State      Link      Status      Address
   * 0 linsrv01        OPEN       eth2      UP           2C:76:8A:56:81:C2
                                          eth3      UP           2C:76:8A:56:81:C3
     1 linsrv02        OPEN
                                            eth2       UP          2C:76:8A:56:85:3F
                                            eth3       UP          2C:76:8A:56:85:3E

What is GAB

GAB is used for group membership services and atomic broadcast.IT maintains cluster membership tracking heartbeat over LLT. If there is no heartbeat over LLT GAB broadcast info to I/O fencing to take further action. It is named GAB due to Group membership and ATOMIC Broadcast.

GAB membership can be displayed using the below command.

[root@linsrv01]# gabconfig -a
GAB Port Memberships
===============================================================
Port a gen   7d6003 membership 01
Port h gen   7d6007 membership 01

So now we have some details about LLT and GAB. These service cab be restarted and started back with the below sequence of commands.This will be a proper way how to stop llt and gab in vcs.

Stopping LLT/GAB

# /sbin/gabconfig –U                    # Unconfigure GAB
# /sbin/lltconfig –U                      # Unconfigure LLT
# modinfo |egrep “gab|llt”        # To determine Ids of GAB/LLT modules.
# modunload –i 305                     # Unload GAB modules.
# modunload –i 292                     # Unload LLT modules.

Starting LLT/GAB

# /etc/rc2.d/S70llt start                           # Restart LLT
# /etc/rc2.d/S92gab start                         # Restart GAB
# hastart                                                  # Start Cluster

VCS Status Validation

# /sbin/gabconfig –a                # Verify GAB & VCS are Started
# /sbin/gabconfig -a                # To display link config info.
# vxdctl -c mode                    # Finds master node when CVM is running
# cat /etc/llttab                      # To verify LLT configuration
# lltstat -n                              # To verify that links are active for LLT.
# lltstat -p                              # To obtain info about ports open for LLT.

If the above method is not working in the scenario if one of your nodes is having critical hardware kinds of issues and you do not have enough node to run the cluster you can do the same using seed number.

Starting and Stopping LLT

# /sbin/gabconfig –c –n                  # Seed number 
# /sbin/gabconfig –U                     # Starting and Stopping GAB
# /sbin/gabdiskhb –l                      # Gab Config

LLT Status/Verification

# /sbin/lltconfig –a list lltstat|lltshow|lltdump               
# hatype –modify                           
# hasys –force system_name          
# hauser –add user_name              
# hacf –verify