Types of failover operations in Hyper-V Replica–Part III – Unplanned Failover

In the first two parts of this 3-part series, you learnt about Test Failover (TFO) and Planned Failover (PFO). In this closing part of the series, I will talk about unplanned failover and summarize the differences of these 3.

1. What is Unplanned Failover?

Unplanned Failover is an operation initiated on the replica VM when the primary VM/site is hit by a disaster. During Unplanned Failover, a check is done using Remote WMI to see if the primary VM is running.This is to protect against accidental administrator actions on the replica VM. This check prevents a ‘split-brain’ scenario where both the production and the replica VMs are running.

2. When should I use Unplanned Failover?

Unplanned Failover is used in the following cases

  • My primary site is experiencing unexpected power outage or a natural disaster
  • My primary site/VM has had a virus attack and I want to restore my business quickly with minimal data loss by restoring my replica VM

3. How should I use Unplanned Failover?

Unplanned failover is performed on the replica virtual machine by right-clicking on the VM and choosing the Failover operation (either from the Hyper-V Manager or from the Failover Clustering Manager).


If you have turned on recovery history, Unplanned Failover can be performed against a previous point-in-time. This is usually done in case the most recent point is either corrupt or not application consistent. Once you failover, you should run some tests to check that the point-in-time is good. If the point-in-time has issues, you can cancel the failover using “Cancel Failover” on the replica VM. Then you can choose a different point-in-time and do a Failover.

After you have validated that the failed over VM is kosher, you should do a ‘Complete’ of the failover by performing an action on the replica virtual machine – this will ensure that the recovery points are merged.

The above procedure can be achieved using Powershell using the following cmdlets. Use Complete-VMFailover only  after ensuring that the failed over VM serves the purpose.

   1: $snapshots = Get-VMSnapshot -VMName VirtualMachine_Workload -SnapshotType Replica
   3: Start-VMFailover -Confirm:$false -VMRecoverySnapshot $snapshots[0]
   5: Complete-VMFailover -VMName VirtualMachine_Workload -Confirm:$false


The table below calls out the characteristics of the 3 failovers


Test Failover

Planned Failover

Unplanned Failover

Operation initiated on

Replica VM

Initiated on the primary VM and completed on the replica VM

Replica VM

Is a duplicate VM created during the operation?




How long is the operation run?


Depends on maintenance window or regulation requirement

Depends on when the primary is brought back up

Recommended frequency

Once a month

Once in 6 months

Never (ok, fine – whenever you have a disasterSmile)

What happens to the replication of the primary VM during the duration of this operation


Continues. In this operation, a role-reversal happens, the primary VM becomes the replica VM and replication continues back to the primary site (that initiated the operation).


Is there data loss?



There can be data loss

Is there down time?


Planned downtime

Unplanned downtime

When to use

  • Run minimal tests to validate if your replication is on track
  • Train your personnel on what is to be done in case of a disaster.
  • Test the recovery plan that you have built to test your preparation when disaster does strike.
  • Perform host maintenance on your primary and would like to run from the replica site.
  • Your primary site is expecting some power outage – you want to move over to the replica site.
  • There’s an impending typhoon – you want to proactively take action to ensure business continuity.
  • Your compliance requirements mandate that every quarter, you run your workloads from the replica site for a week.
  • Your primary site is experiencing unexpected power outage or a natural disaster
  • Your primary site/VM has had a virus attack and you want to restore your business quickly with minimal data loss by restoring your replica VM


In closing, use Test Failover frequently to check the fidelity of your replication and test your recovery plans. Use Planned Failover occasionally for either planned maintenance or disaster simulation or compliance reasons. Use Unplanned Failover when your primary site is hit by a disaster.