6.0.1

Table Of Contents
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If both the Primary VM and Secondary VM cannot access the metadata datastore, the VMs might fail
unexpectedly. Typically, an unrelated failure that terminates FT must also occur when access to the FT
metadata datastore is lost by both VMs. vSphere HA then tries to restart the Primary VM on a host with
access to the metadata datastore.
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The VM might stop being recognized as an FT VM by vCenter Server. This failed recognition can allow
unsupported operations such as taking snapshots to be performed on the VM and cause problematic
behavior.
Cause
Lack of access to the Fault Tolerance metadata datastore can lead to the undesirable outcomes in the
previous list.
Solution
When planning your FT deployment, place the metadata datastore on highly available storage. While FT is
running, if you see that the access to the metadata datastore is lost on either the Primary VM or the
Secondary VM, promptly address the storage problem before loss of access causes one of the previous
problems. If a VM stops being recognized as an FT VM by vCenter Server, do not perform unsupported
operations on the VM. Restore access to the metadata datastore. After access is restored for the FT VMs and
the refresh period has ended, the VMs are recognizable.
Turning On vSphere FT for Powered-On VM Fails
If you try to turn on vSphere Fault Tolerance for a powered-on VM, this operation can fail.
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:
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Free up memory resources on the host to accommodate the VM's memory reservation and the added
overhead.
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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).
vSphere Troubleshooting
16 VMware, Inc.