8000 doc: Add advice about systemd RemoveIPC · intobs/postgres@fbe7a3f · GitHub
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doc: Add advice about systemd RemoveIPC
Reviewed-by: Magnus Hagander <magnus@hagander.net>
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‎doc/src/sgml/runtime.sgml

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</sect2>
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<sect2 id="systemd-removeipc">
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<title>systemd RemoveIPC</title>
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<indexterm>
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<primary>systemd</primary>
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<secondary>RemoveIPC</secondary>
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</indexterm>
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<para>
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If <productname>systemd</productname> is in use, some care must be taken
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that IPC resources (shared memory and semaphores) are not prematurely
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removed by the operating system. This is especially of concern when
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installing PostgreSQL from source. Users of distribution packages of
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PostgreSQL are less likely to be affected, as
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the <literal>postgres</literal> user is then normally created as a system
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user.
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</para>
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<para>
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The setting <literal>RemoveIPC</literal>
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in <filename>logind.conf</filename> controls whether IPC objects are
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removed when a user fully logs out. System users are exempt. This
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setting defaults to on in stock <productname>systemd</productname>, but
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some operating system distributions default it to off.
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</para>
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<para>
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A typical observed effect when this setting is on is that the semaphore
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objects used by a PostgreSQL server are removed at apparently random
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times, leading to the server crashing with log messages like
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<screen>
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LOG: semctl(1234567890, 0, IPC_RMID, ...) failed: Invalid argument
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</screen>
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Different types of IPC objects (shared memory vs. semaphores, System V
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vs. POSIX) are treated slightly differently
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by <productname>systemd</productname>, so one might observe that some IPC
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resources are not removed in the same way as others. But it is not
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advisable to rely on these subtle differences.
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</para>
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<para>
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A <quote>user logging out</quote> might happen as part of a maintenance
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job or manually when an administrator logs in as
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the <literal>postgres</literal> user or something similar, so it is hard
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to prevent in general.
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</para>
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<para>
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What is a <quote>system user</quote> is determined
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at <productname>systemd</productname> compile time from
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the <symbol>SYS_UID_MAX</symbol> setting
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in <filename>/etc/login.defs</filename>.
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</para>
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<para>
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Packaging and deployment scripts should be careful to create
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the <literal>postgres</literal> user as a system user by
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using <literal>useradd -r</literal>, <literal>adduser --system</literal>,
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or equivalent.
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</para>
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<para>
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Alternatively, if the user account was created incorrectly or cannot be
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changed, it is recommended to set
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<programlisting>
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RemoveIPC=no
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</programlisting>
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in <filename>/etc/systemd/logind.conf</filename> or another appropriate
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configuration file.
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</para>
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<caution>
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<para>
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At least one of these two things has to be ensured, or the PostgreSQL
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server will be very unreliable.
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</para>
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</caution>
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</sect2>
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<sect2>
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<title>Resource Limits</title>
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