When you upgrade vCenter Server and the Platform Services Controller, the installer does a pre-upgrade check, for example, to verify that enough space is available on the virtual machine or physical server where you are upgrading vCenter Server, and verifies that the external database, if any, can be successfully accessed.
When you deploy vCenter Server with an embedded Platform Services Controller, or an external Platform Services Controller, vCenter Single Sign-On is installed as part of the Platform Services Controller. At the time of upgrade, the installer provides you with the option to join an existing vCenter Single Sign-On server domain. When you provide the information about the other vCenter Single Sign-On service, the installer uses the administrator account to check the host name and password, to verify that the details of the vCenter Single Sign-On server you provided can be authenticated before proceeding with the upgrade process.
The pre-upgrade checker performs checks for the following aspects of the environment:
Minimum processor requirements
Minimum memory requirements
Minimum disk space requirements
Permissions on the selected install and data directory
Internal and external port availability
External database version
External database connectivity
Administrator privileges on the Windows machine
Any credentials that you enter
vCenter Server 5.x services
For information about the minimum storage requirements, see vCenter Server for Windows Storage Requirements. For information about the minimum hardware requirements, see vCenter Server for Windows Hardware Requirements.