You cannot control the partition usage. But the rollback process is pretty simple in the gui. When I do major release upgrades, I generally create a snapshot and download this prior to the install. This is a fallback in case the upgrade goes badly or needs to be rolled back. The upgrade process itself does create and use these configuration snapshots automatically. But I like to have a manual one as well. In running upgrades across major release, I generally rollback about one in five times because we hit some kind of anomalous behavior. But the most recent moves from 5 to 6 have all gone well on the firewalls. I did hit a bug in Panorama upgrades from 5 to 6 but that was an edge case pretty specific to the configuration.
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