Hi All 
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In the fourteenth part of our series, we'll go through a supplementary feature of vSphere Clustering: Fault Tolerance (FT). This feature is sometimes called Super HA, as it saves a critical VM from any failure as well as it keeps that VM always online with zero downtime when vSphere HA requires VM restart. Although FT is a promising feature, it requires some restricted configuration and hence in this version (5.x) it can't be used widely.
Fault Tolerance (FT) makes a shadow (secondary) copy of VM which replicates with the primary VM with time difference in microseconds. When primary VM fails, the secondary automatically comes in place and user and even applications connected can’t sense this momentarily disconnection/connection transition. The following old-released document by VMware about FT describes it clearly:
http://www.vmware.com/files/pdf/fault_tolerance_recommendations_considerations_on_vmw_vsphere4.pdf
Credits:
- William Lam
Now, Let's Start...
1. Disable Fault Tolerance vs. Turning Fault Tolerance off:
Disabling VMware FT preserves the secondary virtual machines, their configuration, and all history. Use this option if you might re-enable VMware FT in the future. Turning off VMware FT deletes all secondary virtual machines, their configuration, and all history. Use this option if you do not plan to re-enable VMware FT.
2. Max. Number of FT Machines:
Max. number of FT VMs (primary and secondary) on a single host is four, which can be modified using an adv. setting (das.maxftvmsperhost).
3. FT vs. EVC-Enabled Clusters:
EVC feature is required to be enabled to make FT VMs managed by DRS engine or these VMs will have a DRS automation level disabled. That’s because FT VMs are both live VMs and can’t be migrated to a host with different CPU architecture.
4. FT vs. DRS Affinity Rules:
VM-VM affinity rule applies only to the Primary FT VM while VM-Host affinity rule applies to both Primary and Secondary VMs.
5. Enabling FT in Nested-Virtualization Environments:
This nice article by William Lam describes in details how to enable FT in a nested-virtualization environments which can be used in Home Labs. I tested it and went successfully on vSphere 5.1, but I don’t know about vSphere 5.5 (confirmation needed):
http://www.virtuallyghetto.com/2011/07/how-to-enable-nested-vft-virtual-fault.html
Share the Knowledge ...
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