Re: [HACKERS] Restricting maximum keep segments by repslots

Hello,

At Thu, 11 Jan 2018 13:56:14 +0000, Greg Stark <stark(at)mit(dot)edu> wrote in <CAM-w4HOVYZkCbCdFt8N8zwAAcuETFimwOB_Db+jgFajn-iYHEQ(at)mail(dot)gmail(dot)com>
> On 11 January 2018 at 09:55, Sergei Kornilov <sk(at)zsrv(dot)org> wrote:
> > if (active_pid != 0)
> > status = "streaming";
> > else
> > status = "keeping";
>
> Perhaps "idle" by analogy to a pg_stat_activity entry for a backend
> that's connected but not doing anything.

The state "keeping" is "some segments that are needed by a slot
are still existing but to be removed by the next checkpoint". The
three states are alogogous to green/yellow/red in traffic
lights. "idle" doesn't feel right.

> > status = "may_lost";
>
> Perhaps "stale" or "expired"?

Some random thoughts on this topic:

Reading the field as "WAL record at restrat_lsn is/has been
$(status)", "expired" fits there. "safe"/"crtical"/("stale" and
"expired") would fit "restart_lsn is $(status)"?

If we merge the second sate to the red-side, a boolean column
with the names "wal_preserved" or "wal_available" might work. But
I believe the second state is crucial.

> Is this patch in bike-shed territory? Are there any questions about
> whether we want the basic shape to look like this?

FWIW the summary history of this patch follows.

- added monitoring feature,
- GUC in bytes not in segments,
- show the "min_keep_lsn" instead of "spare amount of avalable
WAL(distance)" (*1)
- changed the words to show the status. (still under discussion)
- added documentation.

I didn't adopt "setting per slot" since the keep amount is not
measured from slot's restart_lsn, but from checkpoint LSN.

*1: As I mentioned upthread, I think that at least the
"pg_replication_slots.min_keep_lsn" is arguable since it shows
the same value for all slots and I haven't found no other
appropriate place.

> Fwiw I think there's a real need for this feature so I would like to
> get it in for Postgres 11.

It encourages me a lot. Thanks.

regards,

--
Kyotaro Horiguchi
NTT Open Source Software Center