Re: CLOG extension

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> [ CLOG extension is horrid for concurrency ]

Yeah. When that code was designed, a page's worth of transactions
seemed like a lot so we didn't worry too much about performance glitches
when we crossed a page boundary. It's time to do something about it
though.

The idea of extending CLOG in advance, so that the work doesn't have to
be done with quite so many locks held, sounds like a plan to me. The
one thing I'd worry about is that extension has to interact with
freezing of very old XIDs and subsequent removal of old clog pages;
make sure that pages will get removed before they could possibly
get created again.

> First, do we really need to WAL-log CLOG extension at all? Perhaps
> recovery should simply extend CLOG when it hits a commit or abort
> record that references a page that doesn't exist yet.

Maybe, but see above. I'd be particularly worried about this in a hot
standby situation, as you would then end up with HS queries seeing XIDs
(in tuples) for which there was no clog page yet. I'm inclined to think
it's better to continue to WAL-log it, but try to arrange to do that
without holding the other locks that are now involved.

regards, tom lane