Re: VACUUM always makes all pages dirty

Yeah, it's definitely a HOT-introdued thing. Vacuum calls
heap_page_prune on every page, and this in heap_page_prune is dirtying
the buffer:

>       else
>       {
>               /*
>                * If we didn't prune anything, we have nonetheless updated the
>                * pd_prune_xid field; treat this as a non-WAL-logged hint.
>                */
>               SetBufferCommitInfoNeedsSave(buffer);
>       }

I don't have time to dig deeper at this moment. I'll take a look later
today, unless someone beats me to it. We obviously don't want to call
SetBufferCommitInfoNeedsSave if we didn't really change the pd_prune_xid
field.