Keep in mind that if you upgrade to ESX server, VMKusage will not start running automatically after the upgrade. If you’re upgrading from esx server 2.1.1 or newer, vmkusage will still be installed after an upgrade (and all the databases will be present). But it does not start collecting data. If you run the command vmkusagectl install it will wipe all the databases and you’re back at empty graphs again.
The trick is to copy the file /etc/cron.d/vmkusage-cron.sh to a safe location (e.g. /root) and copy it back after the upgrade has been completed.