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Failback is one of the ways to finalize failover. When you perform failback, you switch back to the production VM from a VM replica, shift I/O processes from the disaster recovery site to the production site.

Veeam Backup & Replication provides you the following options to perform failback:

You can fail back to the original VM in the original location.
You can fail back to a VM already recovered to a new location. This VM must be recovered before you perform failback. For example, you can recover the VM from a backup.
You can fail back to a VM recovered from a replica to a new location, or to any location but with different settings. The VM will be recovered from the replica during the failback process.

The first two options help you decrease recovery time and the use of the network traffic because Veeam Backup & Replication needs to transfer only differences between the original/recovered VM and VM replica. For the third option, Veeam Backup & Replication needs to transfer the whole VM data, including its configuration and virtual disk content. Use the third option if there is no way to use the original VM or restore it from a backup.

Veeam Backup & Replication performs failback in two phases. During the first phase, Veeam Backup & Replication synchronizes the state of the production VM (the original VM, an already recovered VM or a VM that will be recovered from the replica) with the current state of its replica. This phase may take a lot of time especially if VM is large. During the second phase, Veeam Backup & Replication switches all processes from the VM replica to the production VM, turns off the replica and also sends to the production VM changes made to the VM replica since the end of the first phase.

Veeam Backup & Replication performs failback in two phases:

First phase: Veeam Backup & Replication synchronizes the state of the production VM (the original VM, an already recovered VM or a VM that will be recovered from the replica) with the current state of its replica. This phase may take a lot of time especially if VM is large. While Veeam Backup & Replication performs the first phase of failback, VM replicas are still up and running, users can access these VMs and perform daily routine tasks as normal.
Second phase: Veeam Backup & Replication switches all processes from the VM replica to the production VM, turns off the replica and also sends to the production VM changes made to the VM replica since the end of the first phase.

The time when the second phase starts depends on how you want to switch from the replica to the production VM. You can switch to the production VM automatically, at the scheduled time or manually. If you select to switch automatically, the second phase will start right after the first phase finishes. If you select to switch at the scheduled time or manually, the second phase will start at the time you want.

The process of failing back to the original VM or an already recovered VM differs from the process of failing back to a VM recovered from a replica:

How failback to the original VM and already recovered VM works
How failback to a VM recovered from a replica works

How Failback to Original VM or Already Recovered VM Works

When you fail back to the original VM or an already recovered VM, Veeam Backup & Replication performs the following operations during the first phase:

If the original VM is running, Veeam Backup & Replication powers it off.
Veeam Backup & Replication calculates the difference between disks of the production VM and disks of the VM replica in the Failover state. Difference calculation helps Veeam Backup & Replication understand what data needs to be transferred to the production VM to synchronize its state with the state of the VM replica.

If you fail back to the original VM in the original location and you have enabled the Quick rollback option, difference calculation can be performed much faster than without this option enabled. For more information on quick rollback, see Quick Rollback.

Veeam Backup & Replication transfers the data that was detected at the previous step to the production VM. The transferred data is written to the production VM.
Veeam Backup & Replication changes the state of the VM replica from Failover to Ready to switch.

During the second phase, Veeam Backup & Replication performs the following operations:

The guest OS of the VM replica is shut down or the VM replica is powered off.

If VMware Tools are installed on the VM replica, Veeam Backup & Replication tries to shut down the replica guest OS. If nothing happens after 15 minutes, Veeam Backup & Replication powers off the VM replica. If VMware Tools are not installed on the VM or the VM is suspended, Veeam Backup & Replication powers off the VM. The VM replica remains powered off until you commit failback or undo failback.

Veeam Backup & Replication calculates the difference between disks of the production VM and disks of the VM replica. Difference calculation helps Veeam Backup & Replication understand what data was changed while the VM replica was in the Ready to switch state.
Sends data changed on the VM replica while it was in the Ready to switch state to the production VM.
The state of the VM replica is changed from Ready to switch to Failback.
[If you fail back to a recovered VM] Veeam Backup & Replication updates the ID of the original VM in the Veeam Backup & Replication configuration database. The ID of the original VM is replaced with the ID of the recovered VM.
If you have selected to power on the production VM after failback, Veeam Backup & Replication powers on the production VM on the host.

How Failback to VM Recovered from Replica Works

When you fail back to a VM recovered from a replica, Veeam Backup & Replication performs the following operations during the first phase:

Veeam Backup & Replication requests vCenter Server to create on the target host an empty VM with the same configuration as the VM replica. vCenter Server registers the created production VM.
Veeam Backup & Replication transfers data of the VM replica to the production VM to update the production VM state to the VM replica state.
Veeam Backup & Replication changes the state of the VM replica from Failover to the Ready to switch.

During the second phase, Veeam Backup & Replication performs the same operations as described in How Failback to Original VM or Already Recovered VM Works except for the step 2. When you fail back to a VM recovered from a replica, Veeam Backup & Replication does not calculate the difference between disks.

Failback is an intermediate step that needs to be finalized. If the production VM works as expected and you want to get back to it, commit failback. If the VM does not work as expected, undo failback.