6.0.1

For virtual machines with Fault Tolerance enabled, you might use ISO images that are accessible only to the
Primary VM. In such a case, the Primary VM can access the ISO, but if a failover occurs, the CD-ROM
reports errors as if there is no media. This situation might be acceptable if the CD-ROM is being used for a
temporary, noncritical operation such as a patch.
Avoid Network Partitions
A network partition occurs when a vSphere HA cluster has a management network failure that isolates
some of the hosts from vCenter Server and from one another. See “Network Partitions,” on page 21. When a
partition occurs, Fault Tolerance protection might be degraded.
In a partitioned vSphere HA cluster using Fault Tolerance, the Primary VM (or its Secondary VM) could end
up in a partition managed by a master host that is not responsible for the virtual machine. When a failover is
needed, a Secondary VM is restarted only if the Primary VM was in a partition managed by the master host
responsible for it.
To ensure that your management network is less likely to have a failure that leads to a network partition,
follow the recommendations in “Best Practices for Networking,” on page 40.
Using Virtual SAN Datastores
vSphere Fault Tolerance can use Virtual SAN datastores, but you must observe the following restrictions:
n
A mix of Virtual SAN and other types of datastores is not supported for both Primary VMs and
Secondary VMs.
n
Virtual SAN metro clusters are not supported with FT.
To increase performance and reliability when using FT with Virtual SAN, the following conditions are also
recommended.
n
Virtual SAN and FT should use separate networks.
n
Keep Primary and Secondary VMs in separate Virtual SAN fault domains.
Legacy Fault Tolerance
By default, vSphere Fault Tolerance (FT) can accommodate symmetric multiprocessor (SMP) virtual
machines with up to four vCPUs. If your virtual machine has only a single vCPU, however, you can use
legacy FT instead for backward compatibility. Unless technically necessary, use of legacy FT is not
recommended.
To use legacy Fault Tolerance, you must configure an advanced option for the virtual machine. After you
complete this configuration, the legacy FT VM is different in some ways from other fault tolerant VMs.
Differences for VMs That Use Legacy FT
VMs that use FT and VMs that use legacy FT differ in several ways.
Table 3‑2. Differences Between Legacy FT and FT
Legacy FT FT
Extended Page Tables/Rapid
Virtualization Indexing (EPT/RVI)
Not supported Required
IPv6 Not supported for legacy FT logging
NICs.
Supported for FT-logging NICs.
DRS Fully supported for initial placement,
load balancing, and maintenance
mode support.
Only power on placement of
Secondary VM and maintenance mode
are supported.
Chapter 3 Providing Fault Tolerance for Virtual Machines
VMware, Inc. 57