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The Reduced Downtime Upgrade (RDU) is a new mechanism introduced in vSphere 8.x for upgrade/update the self-managed vCenter Servers with a minimal downtime (lesser compared to normal upgrade/update procedure).

What is the Self-managed vCenter Server? It’s a vCenter Server that manages its own ESXi host and its own virtual machine.

More information are available in KB 92659 (vCenter Server upgrades with the reduced downtime).

If you try to update a vCSA 8.0.x with VAMI you will notice this reminder about the new RDU feature:

Actually works only with 8.0.x version, so can be used as an update tool according with the following table:

Source VersionTarget Version
8.0 GA8.0 U2 or later releases
8.0 U18.0 U2 or later releases
8.0 P028.0 U2 or later releases
8.0 U2Releases later than 8.0 U2

The produce to use RDU is a little more complicated compared to traditional update (basically with a single click you start the update).

The new model require to deploy a new VM with a temporary IP where the vCenter Server database and all the configurations are migrated.

Then the different services are started on the new VM and this require a downtime.

After all the old VM will be powered off and the new will acquire the original IP address.