Re: [HACKERS] Restricting maximum keep segments by repslots

At Fri, 15 Feb 2019 19:13:23 -0800, Andres Freund <andres(at)anarazel(dot)de> wrote in <20190216031323(dot)t7tfrae4l6zqtseo(at)alap3(dot)anarazel(dot)de>
> Hi,
>
> On 2019-01-30 10:42:04 +0900, Kyotaro HORIGUCHI wrote:
> > From 270aff9b08ced425b4c4e23b53193285eb2359a6 Mon Sep 17 00:00:00 2001
> > From: Kyotaro Horiguchi <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
> > Date: Thu, 21 Dec 2017 21:20:20 +0900
> > Subject: [PATCH 1/6] Add WAL relief vent for replication slots
> >
> > Adds a capability to limit the number of segments kept by replication
> > slots by a GUC variable.
>
> Maybe I'm missing something, but how does this prevent issues with
> active slots that are currently accessing the WAL this patch now
> suddenly allows to be removed? Especially for logical slots that seems
> not unproblematic?

No matter whether logical or physical, when reading an
overwritten page of a recycled/renamed segment file, page
validation at reading-in finds that it is of a different segment
than expected. 0006 in [1] introduces more active checking on
that.

[1] https://www.postgresql.org/message-id/20181220.162438.121484007.horiguchi.kyotaro%40lab.ntt.co.jp

> Besides that, this patch needs substantial spelling / language / comment
> polishing. Horiguchi-san, it'd probably be good if you could make a
> careful pass, and then maybe a native speaker could go over it?

Thank you for your kind suggestion. As I did for other patches,
I'll review it by myself and come up with a new version soon.

# I often don't understand what I wrote:(

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center