Date: Wed, 27 Feb 2019 13:52:42 +0100
From: Johannes Truschnigg <johannes@truschnigg.info>
To: John Scalia <jayknowsunix@gmail.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: Logical replication and wal segment retention
Message-ID: <20190227125242.6ixy4jkzcuj55cdl@vault.lan>
References: <695C083D-AFB5-431B-B022-ED442CE94794@gmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha512;
	protocol="application/pgp-signature"; boundary="uo4nffbqyiuyy476"
Content-Disposition: inline
In-Reply-To: <695C083D-AFB5-431B-B022-ED442CE94794@gmail.com>
User-Agent: NeoMutt/20170113 (1.7.2)
Precedence: bulk


--uo4nffbqyiuyy476
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

Hi Jay,

On Wed, Feb 27, 2019 at 07:40:26AM -0500, John Scalia wrote:
> Hello, folks,
>=20
> Yesterday, I had a small file system fill up, due to some logical
> replication testing we had been performing. We had been testing IBM=E2=80=
=99s IIDR
> system and apparently it had built a logical replication slot on my serve=
r.
> When the test was completed, nobody removed the slot, so WAL segments
> stopped being dropped. Now I can understand the difficulty separating what
> physical versus logical replication needs from the WAL segments, but as
> logical replication is database specific not cluster wide, this behavior =
was
> a little unexpected, since the WAL segments are cluster wide. Are WAL
> segments going to pile up whenever something drops a logical replication
> connection? I=E2=80=99ve seen it, but it seems like this could be a bad t=
hing.

Since Logical Replication is piggybacked on Physical Replication, you cannot
use the first without having the latter. And yes, what you experienced is o=
ne
of the dangers of using replication slots when having a busy database (i.e.
producing lots of WAL) and a filesystem with little excess space. Under the=
se
circumstances, it is imperative to monitor for (and alert on) anything going
awry with your replication slot consumers, and/or the size of your wal/xlog
directory. It's a feature of replication slots to work that way - but one t=
hat
may end up biting you.

--=20
with best regards:
- Johannes Truschnigg ( johannes@truschnigg.info )

www:   https://johannes.truschnigg.info/
phone: +43 650 2 133337
xmpp:  johannes@truschnigg.info

Please do not bother me with HTML-email or attachments. Thank you.

--uo4nffbqyiuyy476
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----
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=yWFm
-----END PGP SIGNATURE-----

--uo4nffbqyiuyy476--