vSphere High Availability
The vSphere High Availability (HA) feature automatically restarts a VM on the same physical server or a different physical server within an ESXi cluster without manual intervention when a guest operating system or hardware failure is detected. When you create a vSphere HA cluster, a single host is automatically elected as the primary host. The primary host communicates with vCenter Server and monitors the state of all protected virtual machines and the state of the secondary hosts. The primary host monitors the network and datastore’s heartbeat from the secondary hosts and if doesn’t detect both the heartbeats (that is, if the secondary host lost both network connectivity and connectivity to its datastore), it restarts the virtual machines of the failed host on other ESXi hosts in the cluster. Because the feature allows for fast recovery from a failed state without much overhead, it is the most common resiliency option used in the vSphere environment. Figure 18-8 shows vSphere High Availability in action.
Figure 18-8 vSphere High Availability
vSphere Fault Tolerance
The vSphere Fault Tolerance (FT) feature allows a protected virtual machine, called the primary VM, to survive the failure of a host with zero downtime. It creates and maintains an identical virtual machine, called the secondary VM, that runs on another host and is continuously available to replace the failed primary VM if the host running the primary VM fails. The primary VM is continuously replicated to the secondary VM using the VMware Fast Checkpointing system so that the secondary VM can take over at any point, thereby providing fault-tolerant protection. If the host running the secondary VM fails, it is also immediately replaced. Although vSphere FT provides zero downtime, unlike vSphere HA, this option is not used very often due to synchronization limitations and overhead.
Figure 18-9 shows how VMware Fast Checkpointing synchronizes the primary and secondary VMs by following the changes on the primary VM and mirroring them to the secondary VM.
Figure 18-9 vSphere Fault Tolerance