Re: making relfilenodes 56 bits

On Mon, Jul 11, 2022 at 7:22 PM Andres Freund <andres@anarazel.de> wrote:
> I guess I'm not enthused in duplicating the necessary knowledge in evermore
> places. We've forgotten one of the magic incantations in the past, and needing
> to find all the places that need to be patched is a bit bothersome.
>
> Perhaps we could add extract helpers out of durable_rename()?
>
> OTOH, I don't really see what we gain by keeping things out of the critical
> section? It does seem good to have the temp-file creation/truncation and write
> separately, but after that I don't think it's worth much to avoid a
> PANIC. What legitimate issue does it avoid?

OK, so then I think we should just use durable_rename(). Here's a
patch that does it that way. I briefly considered the idea of
extracting helpers, but it doesn't seem worthwhile to me. There's not
that much code in durable_rename() in the first place.

In this version, I also removed the struct padding, changed the limit
on the number of entries to a nice round 64, and made some comment
updates. I considered trying to go further and actually make the file
variable-size, so that we never again need to worry about the limit on
the number of entries, but I don't actually think that's a good idea.
It would require substantially more changes to the code in this file,
and that means there's more risk of introducing bugs, and I don't see
that there's much value anyway, because if we ever do hit the current
limit, we can just raise the limit.

If we were going to split up durable_rename(), the only intelligible
split I can see would be to have a second version of the function, or
a flag to the existing function, that caters to the situation where
the old file is already known to have been fsync()'d.

-- 
Robert Haas
EDB: http://www.enterprisedb.com