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man: document new systemd.volatile=overlay kernel command line option

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Lennart Poettering 2018-12-22 00:51:13 +01:00
parent b23f16283d
commit 13070a70ef
2 changed files with 20 additions and 11 deletions
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9
man/kernel-command-line.xml
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@ -137,10 +137,13 @@
enables fully state-less boots were the vendor-supplied OS is used as shipped, with only default
configuration and no stored state in effect, as <filename>/etc</filename> and <filename>/var</filename> (as
well as all other resources shipped in the root file system) are reset at boot and lost on shutdown. If this
setting is set to <literal>state</literal> the root file system is mounted as usual, however
setting is set to <literal>state</literal> the root file system is mounted read-only, however
<filename>/var</filename> is mounted as a volatile memory file system (<literal>tmpfs</literal>), so that the
system boots up with the normal configuration applied, but all state reset at boot and lost at shutdown. For details,
see
system boots up with the normal configuration applied, but all state reset at boot and lost at shutdown. If
this setting is set to <literal>overlay</literal> the root file system is set up as
<literal>overlayfs</literal> mount combining the read-only root directory with a writable
<literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
nonetheless with all changes being lost at reboot. For details, see
<citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
and
<citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>

22
man/systemd-fstab-generator.xml
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@ -175,19 +175,25 @@
lost at shutdown, as <filename>/etc</filename> and <filename>/var</filename> will be served from the (initially
unpopulated) volatile memory file system.</para>
<para>If set to <option>state</option> the generator will leave the root
directory mount point unaltered, however will mount a <literal>tmpfs</literal> file system to
<filename>/var</filename>. In this mode the normal system configuration (i.e. the contents of
<literal>/etc</literal>) is in effect (and may be modified during system runtime), however the system state
(i.e. the contents of <literal>/var</literal>) is reset at boot and lost at shutdown.</para>
<para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
however will mount a <literal>tmpfs</literal> file system to <filename>/var</filename>. In this mode the normal
system configuration (i.e. the contents of <literal>/etc</literal>) is in effect (and may be modified during
system runtime), however the system state (i.e. the contents of <literal>/var</literal>) is reset at boot and
lost at shutdown.</para>
<para>If this setting is set to <literal>overlay</literal> the root file system is set up as
<literal>overlayfs</literal> mount combining the read-only root directory with a writable
<literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
nonetheless with all changes being lost at reboot.</para>
<para>Note that in none of these modes the root directory, <filename>/etc</filename>, <filename>/var</filename>
or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
<para>Note that enabling this setting will only work correctly on operating systems that can boot up with only
<filename>/usr</filename> mounted, and are able to automatically populate <filename>/etc</filename>, and also
<filename>/var</filename> in case of <literal>systemd.volatile=yes</literal>.</para></listitem>
<para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will only work
correctly on operating systems that can boot up with only <filename>/usr</filename> mounted, and are able to
automatically populate <filename>/etc</filename>, and also <filename>/var</filename> in case of
<literal>systemd.volatile=yes</literal>.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>