Re: Unhappy about API changes in the no-fsm-for-small-rels patch

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2019-05-07 09:34:42 -0400, Tom Lane wrote:
>> I'm inclined to wonder why bother with invals at all.

> But when updating the free space for the first four blocks, we're going
> to either have to do an smgrexists() to check whether somebody
> concurrently created the FSM, or we might not update an existing FSM. An
> inval seems much better.

I do not think sinval messaging is going to be sufficient to avoid that
problem. sinval is only useful to tell you about changes if you first
take a lock strong enough to guarantee that no interesting change is
happening while you hold the lock. We are certainly not going to let
writes take an exclusive lock, so I don't see how we could be certain
that we've seen an sinval message telling us about FSM status change.

This seems tied into the idea we've occasionally speculated about
of tracking relation sizes in shared memory to avoid lseek calls.
If we could solve the problems around that, it'd provide a cheap(er)
way to detect whether an FSM should exist or not.

A different way of looking at it is that the FSM data is imprecise
by definition, therefore it doesn't matter that much if some backend
is slow to realize that the FSM exists. That still doesn't lead me
to think that sinval messaging is a component of the solution though.

regards, tom lane