Re: Hard limit on WAL space used (because PANIC sucks)

On Tue, Jan 21, 2014 at 3:43 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> I personally think this isn't worth complicating the code for.

You're probably right. However, I don't see why the bar has to be very
high when we're considering the trade-off between taking some
emergency precaution against having a PANIC shutdown, and an assured
PANIC shutdown. Heikki said somewhere upthread that he'd be happy with
a solution that only catches 90% of the cases. That is probably a
conservative estimate. The schemes discussed here would probably be
much more effective than that in practice. Sure, you can still poke
holes in them. For example, there has been some discussion of
arbitrarily large commit records. However, this is the kind of thing
just isn't that relevant in the real world. I believe that in practice
the majority of commit records are all about the same size.

I do not believe that the two acceptable outcomes here are either that
we continue to always PANIC shutdown (i.e. do nothing), or promise to
never PANIC shutdown. There is likely to be a third way, which is that
the probability of a PANIC shutdown is, at the macro level, reduced
somewhat from the present probability of 1.0. People are not going to
develop a lackadaisical attitude about running out of disk space on
the pg_xlog partition if we do so. They still have plenty of incentive
to make sure that that doesn't happen.

--
Peter Geoghegan