Q: What is the difference between a planned and unplanned Hyper-V Replica failover?

A: There are two primary types of “real” Hyper-V Replica failover (test failover is for testing purposes only). The two types are planned and unplanned. The difference is as follows:

Planned failover.
Used as part of a “planned” failover and is initiated from the primary Hyper-V virtual machine (VM) via the Planned Failover option on the replication menu.

This process shuts down the primary VM, replicates any pending changes to ensure no data loss, fails over to the replica VM, reverses the replication so changes come in from the reverse direction, then starts the replica VM, which now becomes the primary (while the old primary becomes the replica).

Unplanned failover.
Used as part of a failover that isn't planned and assumes the primary Hyper-V VM isn't available because a disaster has struck (if the primary is available, always use the Planned failover, which is preferred). The failover has to be performed on the replica VM using the Failover option on the replication menu.

When this is performed, a replication of pending changes isn't possible and reverse replication has to be manually enabled, with a resynchronization required, because there's no way to know where the primary and the replica stopped replication.

When starting reverse replication, choose the Do not copy the initial replication option on the Initial Replication page. The VM on the original primary VM can be used, and a block-by-block comparison will be performed to synchronize between the replica VM and the original primary VM. Only the delta content needs to be sent over the network.