vMotion between VMware clusters that are across two different datacenters
Stretched clusters can be configured as two different VMware clusters across two different datacenters/sites connected over a WAN link. And its possible for vmotion to happen between these two Vmware clusters.
So if you are using ‘Write Back One Replica’ policy and if there is any chance that any VM will migrate between clusters, then it is required that VirtuCache be installed in all hosts in both clusters, and their VirtuCache Replication networks be able to talk to each other, else there is a chance of data loss.
Now if you always shutdown the VM and move the VM between clusters then the cache will always be flushed to backend storage, and that will always be a safe operation regardless of whether you have VirtuCache installed or not , or if the replication networks talk to each other or not.