Information Technology Reference
In-Depth Information
With vSphere HA, it's important to understand that there will be an interruption of service.
If a physical host fails, vSphere HA restarts the VM, and while the VM is restarting, the applica-
tions or services provided by that VM are unavailable. For users who need even higher levels
of availability than can be provided using vSphere HA, vSphere Fault Tolerance (FT), which is
described in the next section, can help.
vSphere Fault Tolerance
While vSphere HA provides a certain level of availability for VMs in the event of physical host
failure, this might not be good enough for some workloads. vSphere Fault Tolerance (FT) might
help in these situations.
As we described in the previous section, vSphere HA protects against unplanned physi-
cal server failure by providing a way to automatically restart VMs upon physical host failure.
This need to restart a VM in the event of a physical host failure means that some downtime—
generally less than 3 minutes—is incurred. vSphere FT goes even further and eliminates any
downtime in the event of a physical host failure. Using vLockstep technology that is based on
VMware's earlier “record and replay” functionality, vSphere FT maintains a mirrored secondary
VM on a separate physical host that is kept in lockstep with the primary VM. Everything that
occurs on the primary (protected) VM also occurs simultaneously on the secondary (mirrored)
VM, so that if the physical host for the primary VM fails, the secondary VM can immediately
step in and take over without any loss of connectivity. vSphere FT will also automatically
re-create the secondary (mirrored) VM on another host if the physical host for the secondary
VM fails, as illustrated in Figure 1.4. This ensures protection for the primary VM at all times.
Figure 1.4
vSphere FT provides
protection against
host failures with no
downtime experi-
enced by the VMs.
ESXi host
ESXi host
Logging connection
In the event of multiple host failures—say, the hosts running both the primary and second-
ary VMs failed—vSphere HA will reboot the primary VM on another available server, and
vSphere FT will automatically create a new secondary VM. Again, this ensures protection for
the primary VM at all times.
