What is the vSphere Fault Domain Manager (FDM) agent

FDM agent is a part of vSphere HA to monitor availability of the ESXi host and its VMs and also power operations of that protected VMs in front of failures. After release of vSphere 5.0 one of the greatest changes is happened inside the VMware clustering architecture: Operation of vSphere HA! There is no more AAM agent (Automated Availability Manager) and it was replaced by the FDM agent (Fault Domain Manager), so basically there is some sort of differences, include of:

1. Reduce the time duration of cluster configuration.
2. Introduce the datastore heartbeating to prevent restarting of VMs in case of host isolation. So the HA mechanism does not decide based on only the network situation, now there must be at least two shared datastore to decide "Is really a some sort of failure happened?"
3. No more dependency to the VPXA.
4. To setup an HA cluster, FDM agent is not rely on DNS to work, like the old time with AAM agent
5. There is no more primary/secondary nodes and is changed to the master/slave relationship with an automated election system to choose the master node.

FDM agent on ESXi host is responsible for commmunication with the vCenter server (in each of master or slave nodes). However you should understand one important point about the HA operation: Even if the vCenter is down, HA agent on ESXi hosts will work in response to any host or VM failures, but without the vCenter server you cannot re-configure and re-struct the cluster settings (and also HA as a part of it).  

FDM also supports the Jumbo frames for communication in MTU size larger than 1500 bytes, but be careful when you device to change this value. Because the MTU size must be compatible in every devices in path of the HA failover operation from host to host.

Also sometimes enabling the Lockdown mode in the ESXi can cause to FDM operation interruption. So for the FDM troubleshooting procedure, addition to the reconfigure HA, you may need to disable the Lockdown mode temporary.

Related files to the FDM

• First of all, there is a fdm.log file for recording every events related to the FDM agent operations in the /var/log directory.
• Also fdm-profiler-1.log in the /var/log/vmware/fdm shows the FDM information about the version, build and service PID.
• All of the configuration files include the FDM agent config, cluster settings and list of member nodes in the cluster, is in the following path (right-side image): /etc/opt/vmware/fdm

At last to check the status of FDM VIB package, run the following ESXCLI command: 

esxcli software vib list | grep vmware-fdm