When you upgrade vCenter Server, downtime is required for vCenter Server.
Expect downtime for vCenter Server as follows:
The upgrade requires vCenter Server to be out of production for 40-50 minutes, depending on the size of the database. The database schema upgrade takes approximately 10-15 minutes of this time. This estimate does not include host reconnection after the upgrade.
If Microsoft .NET Framework is not installed on the machine, a reboot is required before starting the vCenter Server installation.
VMware Distributed Resource Scheduler does not work while the upgrade is in progress. VMware HA does work during the upgrade.
Downtime is not required for the ESX/ESXi hosts that vCenter Server is managing, or for virtual machines that are running on the hosts.