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man: explain timesyncd epoch mtime touch files in a bit more detail

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Let's mention what /usr/lib/clock-epoch precisely does, and underline we
only *advance* the time based on it.

Inspired by Darkdragon-001's #23214.

Replaces #23214

(cherry picked from commit 2efddcb24551521b75542043d033e39338207de8)
(cherry picked from commit 1c07e6206cbe15ec2f963a65711c06933b2cb5a1)
(cherry picked from commit 38011797cfad69ea7ccb5577a8625b408281337f)
This commit is contained in:
Lennart Poettering 2023-06-12 14:06:10 +02:00 committed by Luca Boccassi
parent aba5381de1
commit 8facb23dd4
1 changed files with 18 additions and 5 deletions
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23
man/systemd-timesyncd.service.xml
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@ -75,14 +75,27 @@
<term><filename>/var/lib/systemd/timesync/clock</filename></term>
<listitem>
<para>The modification time ("mtime") of this file is updated on each successful NTP synchronization
or after each <varname>SaveIntervalSec=</varname> time interval, as specified in
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
At the minimum, it will be set to the systemd build date. It is used to ensure that the system clock
remains roughly monotonic across reboots, in case no local RTC is available.</para>
<para>The modification time ("mtime") of this file is updated on each successful NTP
synchronization or after each <varname>SaveIntervalSec=</varname> time interval, as specified in
<citerefentry><refentrytitle>timesyncd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
<para>When initializing, the local clock is advanced to the modification time of this file (if the
file timestamp is in the past this adjustment is not made). If the file does not exist yet, the
clock is instead advanced to the modification time of <filename>/usr/lib/clock-epoch</filename> –
if it exists – or to a time derived from the source tree at build time. This mechanism is used to
ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across
reboots, in case no battery-buffered local RTC is available.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><filename>/usr/lib/clock-epoch</filename></term>
<listitem><para>The modification time ("mtime") of this file is used for advancing the system clock
in case <filename>/var/lib/systemd/timesync/clock</filename> does not exist yet, see
above.</para></listitem>
</varlistentry>
<varlistentry>
<term><filename>/run/systemd/timesync/synchronized</filename></term>