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McCain c01.tex V3 - 09/17/2009 12:23pm Page 9
EXPLORING VMWARE VSPHERE 4 9
The VMware HA feature, unlike DRS, does not use the VMotion technology as a means of
migrating servers to another host. In a VMware HA failover situation, there is no anticipation
of failure; it is not a planned outage, and therefore there is no time to perform a VMotion.
VMware HA is intended to address unplanned downtime because of the failure of a physical
ESX/ESXi host.
By default VMware HA does not provide failover in the event of a guest operating system
failure, although you can configure VMware HA to monitor virtual machines and restart them
automatically if they fail to respond to an internal heartbeat. For users who need even higher
levels of availability, VMware Fault Tolerance (FT), which is described in the next section, can
satisfy that need.
Chapter 11, ‘‘Ensuring High Availability and Business Continuity,’’ explores the configuration
and working details of VMware High Availability and VMware Fault Tolerance.
VMware Fault Tolerance
For users who require even greater levels of high availability than VMware HA can provide,
VMware vSphere introduces a new feature known as VMware Fault Tolerance (FT).
VMware HA protects against unplanned physical server failure by providing a way to auto-
matically restart virtual machines upon physical host failure. This need to restart a virtual machine
in the event of a physical host failure means that some downtime—generally less than three
minutes—is incurred. VMware FT goes even further and eliminates any downtime in the event of
a physical host failure. Using vLockstep technology, VMware 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 on which the primary VM is running fails, the secondary VM can
immediately step in and take over without any loss of connectivity. VMware FT will also auto-
matically re-create the secondary (mirrored) VM on another host if the physical host on which the
secondary VM is running fails, as illustrated in Figure 1.5. This ensures protection for the primary
VM at all times.
Figure 1.5
VMware FT provides
protection against host
failures with no down-
time to the virtual
machines.
No Downtime
ESX/ESXi ESX/ESXi
In the event of multiple host failures—say, the hosts running both the primary and secondary
VMs failed—VMware HA will reboot the primary VM on another available server, and VMware
FT will automatically create a new secondary VM. Again, this ensures protection for the primary
VM at all times.
VMware FT can work in conjunction with VMotion, but it cannot work with DRS, so DRS must
be manually disabled on VMs that are protected with VMware FT.
Chapter 10 provides more information on how to disable DRS for specific VMs. Chapter 11
provides more information on VMware FT.