Robert Haas wrote:
> On Thu, Aug 25, 2016 at 3:21 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
> > Does it work to set the minimum to one WAL segment, i.e. 64MB? guc.c
> > has a hardcoded minimum of 2, but I couldn't find an explanation for it.
>
> Well, I think that when you overrun the end of one segment, you're
> never going to be able to wrap around to the start of the same
> segment; you're going to get sucked into needing another file.
Sure, but that's a transient situation; after a couple of checkpoints,
the old segment can be removed without any danger, leaving only the
active segment. [thinks] Ah, on reflection, there's no way that this
buys anything: it is always critical to have enough disk space to have
one more segment to switch to. So even if you're on tight disk
constraints, you cannot afford to allocate space for a single segment
only, because if you only have that and the need comes to create the
next one to switch to, you will just not have the space.
If we were to use the WAL space in a way different than the POSIX
file interface, we could probably do better. But that seems too
onerous.
I suppose the only option is to keep the minimum at 2. I don't see any
point in forcing the minimum to be more than that, however.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
