Wow, OK. In my view, that makes the chain conversion code pretty much essential, because if you had WARM without chain conversion then the visibility map gets more or less irrevocably less effective over time, which sounds terrible.
Yes. I decided to complete chain conversion patch when I realised that IOS will otherwise become completely useful if large percentage of rows are updated just once. So I agree. It's not an optional patch and should get in with the main WARM patch.
But it sounds to me like even with the chain conversion, it might take multiple vacuum passes before all visibility map bits are set, which isn't such a great property (thus e.g. fdf9e21196a6f58c6021c967dc5776a16190f295).
The chain conversion algorithm first converts the chains during vacuum and then checks if the page can be set all-visible. So I'm not sure why it would take multiple vacuums before a page is set all-visible. The commit you quote was written to ensure that we make another attempt to set the page all-visible after al dead tuples are removed from the page. Similarly, we will convert all WARM chains to HOT chains and then check for all-visibility of the page.
