You can use vSphere Fault Tolerance with vSphere Distributed Resource Scheduler (DRS) when the Enhanced vMotion Compatibility (EVC) feature is enabled. This process allows fault tolerant virtual machines to benefit from better initial placement and also to be included in the cluster's load balancing calculations.
When a cluster has EVC enabled, DRS makes the initial placement recommendations for fault tolerant virtual machines, moves them during cluster load rebalancing, and allows you to assign a DRS automation level to Primary VMs (the Secondary VM always assumes the same setting as its associated Primary VM.)
DRS does not place more than a fixed number of Primary or Secondary VMs on a host during initial placement or load balancing. This limit is controlled by the advanced option das.maxftvmsperhost. The default value for this option is 4. However if you set this option to 0, DRS ignores this restriction.
When vSphere Fault Tolerance is used for virtual machines in a cluster that has EVC disabled, the fault tolerant virtual machines are given DRS automation levels of "disabled". In such a cluster, each Primary VM is powered on only on its registered host, its Secondary VM is automatically placed, and neither fault tolerant virtual machine is moved for load balancing purposes.
If you use affinity rules with a pair of fault tolerant virtual machines, a VM-VM affinity rule applies to the Primary VM only, while a VM-Host affinity rule applies to both the Primary VM and its Secondary VM. If a VM-VM affinity rule is set for a Primary VM, DRS attempts to correct any violations that occur after a failover (that is, after the Primary VM effectively moves to a new host).