I think we need something for pg_upgrade to rewrite existing VMs. Otherwise a large read only database would suddenly require a massive revacuum after upgrade, which seems bad. That can wait for now until we all agree this patch is sound.
Since we need to rewrite the "vm" map, I think we should call the new map "vfm"
That way we will be able to easily check whether the rewrite has been conducted on all relations.
Since the maps are just bits there is no other way to tell that a map has been rewritten
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services