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man: document what "in-memory" units means

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Fixes: #10338
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Lennart Poettering 2018-10-29 21:09:57 +01:00
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man/systemd.xml
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@ -392,6 +392,25 @@
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry> <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>
for details about these target units.</para> for details about these target units.</para>
<para>systemd only keeps a minimal set of units loaded into memory. Specifically, the only units that are kept
loaded into memory are those for which at least one of the following conditions is true:</para>
<orderedlist>
<listitem><para>It is in an active, activating, deactivating or failed state (i.e. in any unit state except for <literal>dead</literal>)</para></listitem>
<listitem><para>It has a job queued for it</para></listitem>
<listitem><para>It is a dependency of some sort of at least one other unit that is loaded into memory</para></listitem>
<listitem><para>It has some form of resource still allocated (e.g. a service unit that is inactive but for which
a process is still lingering that ignored the request to be terminated)</para></listitem>
<listitem><para>It has been pinned into memory programmatically by a D-Bus call</para></listitem>
</orderedlist>
<para>systemd will automatically and implicitly load units from disk — if they are not loaded yet — as soon as
operations are requested for them. Thus, in many respects, the fact whether a unit is loaded or not is invisible to
clients. Use <command>systemctl list-units --all</command> to comprehensively list all units currently loaded. Any
unit for which none of the conditions above applies is promptly unloaded. Note that when a unit is unloaded from
memory its accounting data is flushed out too. However, this data is generally not lost, as a journal log record
is generated declaring the consumed resources whenever a unit shuts down.</para>
<para>Processes systemd spawns are placed in individual Linux <para>Processes systemd spawns are placed in individual Linux
control groups named after the unit which they belong to in the control groups named after the unit which they belong to in the
private systemd hierarchy. (see <ulink private systemd hierarchy. (see <ulink