Re: [HACKERS] Restricting maximum keep segments by repslots

On 2018-07-31 15:21:27 -0400, Stephen Frost wrote:
> Greetings,
> 
> * Andres Freund (andres@anarazel.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.

Greetings,

Andres Freund