<sect2 id="systemd-removeipc">

<title>systemd RemoveIPC</title>

<indexterm>

<primary>systemd</primary>

<secondary>RemoveIPC</secondary>

</indexterm>

<para>

If <productname>systemd</productname> is in use, some care must be taken

that IPC resources (shared memory and semaphores) are not prematurely

removed by the operating system. This is especially of concern when

installing PostgreSQL from source. Users of distribution packages of

PostgreSQL are less likely to be affected, as

the <literal>postgres</literal> user is then normally created as a system

user.

</para>

<para>

The setting <literal>RemoveIPC</literal>

in <filename>logind.conf</filename> controls whether IPC objects are

removed when a user fully logs out. System users are exempt. This

setting defaults to on in stock <productname>systemd</productname>, but

some operating system distributions default it to off.

</para>

<para>

A typical observed effect when this setting is on is that the semaphore

objects used by a PostgreSQL server are removed at apparently random

times, leading to the server crashing with log messages like

<screen>

LOG: semctl(1234567890, 0, IPC_RMID, ...) failed: Invalid argument

</screen>

Different types of IPC objects (shared memory vs. semaphores, System V

vs. POSIX) are treated slightly differently

by <productname>systemd</productname>, so one might observe that some IPC

resources are not removed in the same way as others. But it is not

advisable to rely on these subtle differences.

</para>

<para>

A <quote>user logging out</quote> might happen as part of a maintenance

job or manually when an administrator logs in as

the <literal>postgres</literal> user or something similar, so it is hard

to prevent in general.

</para>

<para>

What is a <quote>system user</quote> is determined

at <productname>systemd</productname> compile time from

the <symbol>SYS_UID_MAX</symbol> setting

in <filename>/etc/login.defs</filename>.

</para>

<para>

Packaging and deployment scripts should be careful to create

the <literal>postgres</literal> user as a system user by

using <literal>useradd -r</literal>, <literal>adduser --system</literal>,

or equivalent.

</para>

<para>

Alternatively, if the user account was created incorrectly or cannot be

changed, it is recommended to set

<programlisting>

RemoveIPC=no

</programlisting>

in <filename>/etc/systemd/logind.conf</filename> or another appropriate

configuration file.

</para>

<caution>

<para>

At least one of these two things has to be ensured, or the PostgreSQL

server will be very unreliable.

</para>

</caution>

</sect2>