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 a minimum of 40 to 50 minutes, and can take much longer depending on the size of the database. The database schema upgrade takes approximately 10 to 15 minutes of this time. This estimate does not include host reconnection time after the upgrade.
For vCenter Server deployments with an embedded database, the upgrade can require extra time to migrate the data from the legacy vCenter Server database to the new database instance.
If Microsoft .NET Framework is not installed on the machine, a restart is required before starting the vCenter Server installation.
vSphere Distributed Resource Scheduler (DRS) does not work while the upgrade is in progress. vSphere HA does work during the upgrade.
Downtime is not required for the ESXi hosts that vCenter Server is managing, or for virtual machines that are running on the hosts.