6.5.1

Table Of Contents
Problem
When you select Turn On Fault Tolerance for a powered-on VM, the operation fails and you see an
Unknown error message.
Cause
This operation can fail if the host that the VM is running on has insufficient memory resources to provide
fault tolerant protection. vSphere Fault Tolerance automatically tries to allocate a full memory reservation
on the host for the VM. Overhead memory is required for fault tolerant VMs and can sometimes expand to
1 to 2 GB. If the powered-on VM is running on a host that has insufficient memory resources to
accommodate the full reservation plus the overhead memory, trying to turn on Fault Tolerance fails.
Subsequently, the Unknown error message is returned.
Solution
Choose from these solutions:
n
Free up memory resources on the host to accommodate the VM's memory reservation and the added
overhead.
n
Move the VM to a host with ample free memory resources and try again.
FT Virtual Machines not Placed or Evacuated by vSphere DRS
FT virtual machines in a cluster that is enabled with vSphere DRS do not function correctly if
Enhanced vMotion Compatibility (EVC) is currently disabled.
Problem
Because EVC is a prerequisite for using DRS with FT VMs, DRS does not place or evacuate them if EVC
has been disabled (even if it is later reenabled).
Cause
When EVC is disabled on a DRS cluster, a VM override that disables DRS on an FT VM might be added.
Even if EVC is later reenabled, this override is not canceled.
Solution
If DRS does not place or evacuate FT VMs in the cluster, check the VMs for a VM override that is
disabling DRS. If you find one, remove the override that is disabling DRS.
Note For more information on how to edit or delete VM overrides, see vSphere Resource Management.
Fault Tolerant Virtual Machine Failovers
A Primary or Secondary VM can fail over even though its ESXi host has not crashed. In such cases,
virtual machine execution is not interrupted, but redundancy is temporarily lost. To avoid this type of
failover, be aware of some of the situations when it can occur and take steps to avoid them.
vSphere Troubleshooting
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