Something you might need to consider when using the VMware Fault Tolerance (FT) feature is how it can have a very big impact on the quantity of machines your cluster can run.
When you enable FT for a VM, a reservation will be made to the size of the memory associated with that VM. So if your virtual machine has 4GB vRAM then the reservation will also be 4GB.
This is all fine and dandy provided it’s not sitting within a VMware cluster with HA enabled using the admission control policy ‘number of host failures to tolerate’. Because this cluster setting uses a slot based algorithm to decide how many virtul machines can be powered up based on the largest memory and CPU reservation, having a huge memory reservation set for an FT protected VM will reduce the number of slots available.
Best thing to do is either change the slot size which you can achieve through the advanced settings or change the HA admission control setting to use ‘Percentage of cluster resources reserved as failover spare capaciy’.
A VMware kb article: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1012602 provides more info on changing the slot setting.
