Automatic free space map filling

Something came to my mind today, I'm not sure if it's feasible but I
would like to know opinions on it.

We've seen database applications that PostgreSQL simply could not manage
because one would have to vacuum continuously. Perhaps in those
situations one could arrange it that an update (or delete) of a row
registers the space in the free space map right away, on the assumption
that by the time it is up for reuse, the transaction will likely have
committed. Naturally, this would need to be secured in some way, for
example a "maybe" bit in the FSM itself or simply checking that the
supposed free space is really free before using it, perhaps combined
with a timeout ("don't consider until 5 seconds from now").

I think with applications that have a more or less constant data volume
but update that data a lot, this could assure constant disk space usage
(even if it's only a constant factor above the ideal usage) without any
vacuuming.

Comments?

--
Peter Eisentraut
http://developer.postgresql.org/~petere/