vSphere5 comes with a completely new HA architecture. There is no more AAM agent like in vSphere 4.1. Instead, there has been a new agent introduced which is named FDM – Fault Domain Manager. The Primary/Secondary concept with 5 primary nodes which has been known in vSphere 4, is gone. You no longer needs to worry not to loose all those 5 primary nodes at the same time and loose the HA functionality for the rest of the cluster. Now there is only one agent in the cluster which plays the role of Master. The agent is called FDM – Fault Domain Manager. One host takes the role of Master. The other agents on other hosts plays only roles as a Slaves, and can became Masters in case the master fails.

The master monitors the availability of ESXi 5 hosts and also the VM availability. The master agent also monitors all slave hosts and in case this slave host fails, all VMs on that host are restarted on another host.  Within each individual host the status of each protected VM is monitored and if a failure of that protected VMs happens, the master proceeds with the restart of this VM. The FDM master keeps a list of VMs being protected, which is updated after every power off or power on status initiated by user. FDM master keeps track of all hosts being a members of a cluster, any adding/removing of hosts refresh this list as well.

When the master fails there is a re-ellection process (this was not the case in vSphere 4) and the host which has an access to the greatest number of datastores is elected as a master. Why's that? It’s because the secondary communication channel is through datastores. The next consideration is the MoID number of a host.

The hosts with slave roles maintain a direct point-to-point TCP connection (no broadcasts) which is encrypted, with the Master. The election process is done via UDP, and then again only via SSL encrypted TCP the communication between the Master and the slaves are maintained.

The host with the master role sends periodically reports states to vCenter. The slaves are informed that the Master is alive via heartbeats. The slaves monitors the state of their locally running VMs and any changes are transmitted to Master. The Slave sends a heartbeats to master and if master should fail, the re-election process occurs. vCenter knows if a new Master is elected, because it’s the new master which contacts vCenter after the re-election process is finished.

The secondary channel through datastores is known as a Heartbeat Datastores.  But this secondary network is not used in normal situations, only in case the primary network goes down. This secondary channel permits the Master to be aware of all Slave hosts and also the VMs running on those hosts. The Heartbeat datastores can also determine if host became isolated or network partitioned. The secondary channel can determine if host is failed (PSOD) or if it’s just isolated. HA likes you to have at least 2 shared datastores for each ESXi host, but you can enable it with just one shared storage but you will get a warning message on the host's front page in the VI Client.

How does this heartbeating mechanism work? HA leverages the existing VMFS filesystem locking mechanism. The locking mechanism uses a so called “heartbeat region” which is updated as long as the lock on a file exists. In order to update a datastore heartbeat region, a host needs to have at least one open file on the volume. HA ensures there is at least one file open on this volume by creating a file specifically for datastore heartbeating. In other words, a per-host a file is created on the designated heartbeating datastores, as shown in the screenshot below. HA will simply check whether the heartbeat region has been updated.

So in my opinion for this to work you need to have your iSCSI and NAS network physically seperated from your ESXi management network containing the heartbeat. FC networks are seperate by default offcourse.

In case of a ESXi host going down while a re-election process is going on, the VMs on this host will be restarted as soon as the new master is elected.

In the scenario of when a Geo-Dispersed cluster is split in two sites due to a link failure, each “partition” will get its own master. Still only 1 master will communicate with vCenter so the data reflected by vCenter might not be 100% accurate.

FDM will work with vCenter5 and ESXi4 hosts. FDM will replace the AAM agent on ESXi4 hosts. The HA concept completely different from ESX4 and before. For instance enabling HA on 32 nodes will take a minute or so to configure, the HA agent is pushed out in parallel instead of serial with 4.1 and prior.

One more important thing: HA no longer uses DNS  – it means there is no dependency on DNS or hosts files.