Re: patch to allow disable of WAL recycling

At Tue, 17 Jul 2018 21:01:03 -0400, Robert Haas <robertmhaas(at)gmail(dot)com> wrote in <CA+Tgmob0hs=eZ7RquTLzYUwAuHtgORvPxjNXgifZ04he-JK7Rw(at)mail(dot)gmail(dot)com>
> On Tue, Jul 17, 2018 at 3:12 PM, Peter Eisentraut
> <peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> > The actual implementation could use another round of consideration. I
> > wonder how this should interact with min_wal_size. Wouldn't
> > min_wal_size = 0 already do what we need (if you could set it to 0,
> > which is currently not possible)?
>
> Hmm, would that actually disable recycling, or just make it happen only rarely?

It doens't. Instead setting max_wal_size smaller than checkpoint
interval should do that.

While considering this, I found a bug in 4b0d28de06, which
removed prior checkpoint from control file. It actually trims the
segments before the last checkpoint's redo segment but recycling
is still considered based on the *prevous* checkpoint. As the
result min_wal_size doesn't work as told. Specifically, setting
min/max_wal_size to 48MB and advance four or more segments then
two checkpoints leaves just one segment, which is less than
min_wal_size.

The attached patch fixes that. One arguable point on this would
be the removal of the behavior when RemoveXLogFile(name,
InvalidXLogRecPtr, ..).

The only place calling the function with the parameter is
timeline switching. Previously unconditionally 10 segments are
recycled after switchpoint but the reason for the behavior is we
didn't have the information on previous checkpoint at hand at the
time. But now we can use the timeline switch point as the
approximate of the last checkpoint's redo point and this allows
us to use min/max_wal_size properly at the time.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center