man: add a note about "archived" journal files and when files can be copied

Issue #6673 requests advice on backup strategy. But the right backup strategy
depends on many factors, too many to describe in a man page. So let's just
provide some general information which files are mutable and that it is always
safe to use/copy files.

Closes #6673.

diff --git a/man/systemd-journald.service.xml b/man/systemd-journald.service.xml
index 8ca0e896abf50aae0689b97c46b4b2707f7ceae8..dfff107e1f15d2e404e0db3d0c238d00ba84dafe 100644 (file)
--- a/man/systemd-journald.service.xml
+++ b/man/systemd-journald.service.xml
@@ -261,9 +261,7 @@ systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
       <varlistentry>
         <term><filename>/etc/systemd/journald.conf</filename></term>
 
-        <listitem><para>Configure
-        <command>systemd-journald</command>
-        behavior. See
+        <listitem><para>Configure <command>systemd-journald</command> behavior. See
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
         </para></listitem>
       </varlistentry>
@@ -274,8 +272,7 @@ systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 
-        <listitem><para><command>systemd-journald</command> writes
-        entries to files in
+        <listitem><para><command>systemd-journald</command> writes entries to files in
         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
         or
         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
@@ -287,7 +284,19 @@ systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
         <filename>/var/log/journal</filename> is not available, or
         when <option>Storage=volatile</option> is set in the
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-        configuration file.</para></listitem>
+        configuration file.</para>
+
+        <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
+        it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
+        (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
+        Such files are "archived" and will not be written to any more.</para>
+
+        <para>In general, it is safe to read or copy any journal file (active or archived).
+        <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+        and the functions in the
+        <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+        library should be able to read all entries that have been fully written.</para>
+        </listitem>
       </varlistentry>
 
       <varlistentry>