by lunarg on January 11th 2019, at 10:04

In the event of migrating your old vCenter Server to a new version (or from Windows to the appliance), it may become necessary to first clear out old historical data. Not only will this speed up the migration process considerably, it will also prevent certain issues which may block the migration from completing successfully.

VMWare provided a KB with database scripts which allow you to selectively purge historical data and decreasing the database size: KB 2110031.