Re: [HACKERS] Restricting maximum keep segments by repslots

At Tue, 31 Jul 2018 12:24:13 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in <20180731192413(dot)7lr4qbc4qbyoim5y(at)alap3(dot)anarazel(dot)de>
> On 2018-07-31 15:21:27 -0400, Stephen Frost wrote:
> > Greetings,
> >
> > * Andres Freund (andres(at)anarazel(dot)de) wrote:
> > > On 2018-07-31 15:11:52 -0400, Bruce Momjian wrote:
> > > > On Tue, Jun 26, 2018 at 04:26:59PM +0900, Kyotaro HORIGUCHI wrote:
> > > > > Hello. This is the reabased version of slot-limit feature.
> > > > >
> > > > > This patch limits maximum WAL segments to be kept by replication
> > > > > slots. Replication slot is useful to avoid desync with replicas
> > > > > after temporary disconnection but it is dangerous when some of
> > > > > replicas are lost. The WAL space can be exhausted and server can
> > > > > PANIC in the worst case. This can prevent the worst case having a
> > > > > benefit from replication slots using a new GUC variable
> > > > > max_slot_wal_keep_size.
> > > >
> > > > Have you considered just using a boolean to control if max_wal_size
> > > > honors WAL preserved by replication slots, rather than creating the new
> > > > GUC max_slot_wal_keep_size?
> > >
> > > That seems like a bad idea. max_wal_size influences checkpoint
> > > scheduling - there's no good reason to conflate that with retention?
> >
> > I agree that we shouldn't conflate checkpointing and retention. What I
> > wonder about though is what value will wal_keep_segments have once this
> > new GUC exists..? I wonder if we could deprecate it...
>
> Don't think that's a good idea. It's entirely conceivable to have a
> wal_keep_segments much lower than max_slot_wal_keep_size. For some
> throwaway things it can be annoying to have to slots, and if you remove
> wal_keep_segments there's no alternative.

I thought it's to be deprecated for some reason so I'm leaving
wal_keep_segments in '# of segments' even though the new GUC is
in MB. I'm a bit uneasy that the two similar settings are in
different units. Couldn't we turn it into MB taking this
opportunity if we will keep wal_keep_segments, changing its name
to min_wal_keep_size? max_slot_wal_keep_size could be changed to
just max_wal_keep_size along with it.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center