man/systemd-journald.service.xml - systemd - Git at Google

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 <refentry id="systemd-journald.service">

   <refentryinfo>
     <title>systemd-journald.service</title>
     <productname>systemd</productname>
   </refentryinfo>

   <refmeta>
     <refentrytitle>systemd-journald.service</refentrytitle>
     <manvolnum>8</manvolnum>
   </refmeta>

   <refnamediv>
     <refname>systemd-journald.service</refname>
     <refname>systemd-journald.socket</refname>
     <refname>systemd-journald-dev-log.socket</refname>
     <refname>systemd-journald-audit.socket</refname>
     <refname>systemd-journald@.service</refname>
     <refname>systemd-journald@.socket</refname>
     <refname>systemd-journald-varlink@.socket</refname>
     <refname>systemd-journald</refname>
     <refpurpose>Journal service</refpurpose>
   </refnamediv>

   <refsynopsisdiv>
     <para><filename>systemd-journald.service</filename></para>
     <para><filename>systemd-journald.socket</filename></para>
     <para><filename>systemd-journald-dev-log.socket</filename></para>
     <para><filename>systemd-journald-audit.socket</filename></para>
     <para><filename>systemd-journald@.service</filename></para>
     <para><filename>systemd-journald@.socket</filename></para>
     <para><filename>systemd-journald-varlink@.socket</filename></para>
     <para><filename>/usr/lib/systemd/systemd-journald</filename></para>
   </refsynopsisdiv>

   <refsect1>
     <title>Description</title>

     <para><filename>systemd-journald</filename> is a system service
     that collects and stores logging data. It creates and maintains
     structured, indexed journals based on logging information that is
     received from a variety of sources:</para>

     <itemizedlist>
       <listitem><para>Kernel log messages, via kmsg</para></listitem>

       <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
       project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
       call</para></listitem>

       <listitem><para>Structured system log messages via the native Journal API, see
       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>
       and <ulink url="https://systemd.io/JOURNAL_NATIVE_PROTOCOL">Native Journal
       Protocol</ulink></para></listitem>

       <listitem><para>Standard output and standard error of service units. For further details see
       below.</para></listitem>

       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
     </itemizedlist>

     <para>The daemon will implicitly collect numerous metadata fields
     for each log messages in a secure and unfakeable way. See
     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
     for more information about the collected metadata.
     </para>

     <para>Log data collected by the journal is primarily text-based but can also include binary data where
     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>

     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
     to volatile storage otherwise. Use <varname>Storage=</varname> in
     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>

     <para>Note that journald will initially use volatile storage, until a call to
     <command>journalctl --flush</command> (or sending <constant>SIGUSR1</constant> to journald) will cause
     it to switch to persistent logging (under the conditions mentioned above). This is done automatically
     on boot via <literal>systemd-journal-flush.service</literal>.</para>

     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
     ensure it has the correct access modes and ownership:</para>

     <programlisting>mkdir -p /var/log/journal
 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>

     <para>See
     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
     for information about the configuration of this service.</para>
   </refsect1>

   <refsect1>
     <title>Stream logging</title>

     <para>The systemd service manager invokes all service processes with standard output and standard error connected
     to the journal by default. This behaviour may be altered via the
     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>

     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
     write attempts will also result in such process signals being generated, see
     <citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
     To mitigate this issue, systemd service manager explicitly turns off the <constant>SIGPIPE</constant>
     signal for all invoked processes by default (this may be changed for each unit individually via the
     <varname>IgnoreSIGPIPE=</varname> option, see
     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
     details). After the standard output/standard error streams have been terminated they may not be recovered
     until the services they are associated with are restarted. Note that during normal operation,
     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in
     the service manager. If <filename>systemd-journald.service</filename> is restarted using
     <command>systemctl restart</command> or equivalent operation instead of a pair of separate
     <command>systemctl stop</command> and <command>systemctl start</command> commands (or equivalent
     operations), these stream connections are not terminated and survive the restart. It is thus safe to
     restart <filename>systemd-journald.service</filename>, but stopping it is not recommended.</para>

     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
     processes (such as further child processes forked off the main service process), the log records will not reflect
     their metadata, but will continue to describe the original process. This is different from the other logging
     transports listed above, which are inherently record based and where the metadata is always associated with the
     individual record.</para>

     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
     line tool.</para>

     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
     4096. When this limit is reached further log streams may be established but will receive
     <constant>EPIPE</constant> right from the beginning.</para>
   </refsect1>

   <refsect1>
     <title>Journal Namespaces</title>

     <para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
     consisting of one or more services from the rest of the system and a mechanism for improving
     performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
     stream managed by its own instance of <command>systemd-journald</command>. Namespaces are independent of
     each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
     exists, managed by <filename>systemd-journald.service</filename> (and its associated socket
     units). Additional namespaces are created by starting an instance of the
     <filename>systemd-journald@.service</filename> service template. The instance name is the namespace
     identifier, which is a short string used for referencing the journal namespace. Service units may be
     assigned to a specific journal namespace through the <varname>LogNamespace=</varname> unit file setting,
     see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
     details. The <option>--namespace=</option> switch of
     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
     used to view the log stream of a specific namespace. If the switch is not used the log stream of the
     default namespace is shown, i.e. log data from other namespaces is not visible.</para>

     <para>Services associated with a specific log namespace may log via syslog, the native logging protocol
     of the journal and via stdout/stderr; the logging from all three transports is associated with the
     namespace.</para>

     <para>By default only the default namespace will collect kernel and audit log messages.</para>

     <para>The <command>systemd-journald</command> instance of the default namespace is configured through
     <filename>/etc/systemd/journald.conf</filename> (see below), while the other instances are configured
     through <filename>/etc/systemd/journald@<replaceable>NAMESPACE</replaceable>.conf</filename>. The journal
     log data for the default namespace is placed in
     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable></filename> (see below) while the data
     for the other namespaces is located in
     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable>.<replaceable>NAMESPACE</replaceable></filename>.</para>
   </refsect1>

   <refsect1>
     <title>Signals</title>

     <variablelist>
       <varlistentry>
         <term>SIGUSR1</term>

         <listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
         <filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
         after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run/</filename> is
         never flushed to <filename>/var/</filename> regardless of the configuration. Use the
         <command>journalctl --flush</command> command to request flushing of the journal files, and wait for
         the operation to complete. See
         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
         details.</para></listitem>
       </varlistentry>

       <varlistentry>
         <term>SIGUSR2</term>

         <listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
         --rotate</command> command to request journal file rotation, and wait for the operation to
         complete.</para></listitem>
       </varlistentry>

       <varlistentry>
         <term>SIGRTMIN+1</term>

         <listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
         --sync</command> command to trigger journal synchronization, and wait for the operation to
         complete.</para></listitem>
       </varlistentry>
     </variablelist>
   </refsect1>

   <refsect1>
     <title>Kernel Command Line</title>

     <para>A few configuration parameters from
     <filename>journald.conf</filename> may be overridden on the kernel
     command line:</para>

     <variablelist class='kernel-commandline-options'>
       <varlistentry>
         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
         <term><varname>systemd.journald.forward_to_console=</varname></term>
         <term><varname>systemd.journald.forward_to_wall=</varname></term>

         <listitem><para>Enables/disables forwarding of collected log
         messages to syslog, the kernel log buffer, the system console
         or wall.
         </para>

         <para>See
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
         for information about these settings.</para>
         </listitem>

       </varlistentry>
     </variablelist>

     <para>Note that these kernel command line options are only honoured by the default namespace, see
     above.</para>
   </refsect1>

   <refsect1>
     <title>Access Control</title>

     <para>Journal files are, by default, owned and readable by the
     <literal>systemd-journal</literal> system group but are not
     writable. Adding a user to this group thus enables them to read
     the journal files.</para>

     <para>By default, each user, with a UID outside the range of system users,
     dynamic service users, and the nobody user, will get their own set of
     journal files in <filename>/var/log/journal/</filename>. See
     <ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
     for more details about UID ranges. These journal
     files will not be owned by the user, however, in order to avoid
     that the user can write to them directly. Instead, file system
     ACLs are used to ensure the user gets read access only.</para>

     <para>Additional users and groups may be granted access to journal
     files via file system access control lists (ACL). Distributions
     and administrators may choose to grant read access to all members
     of the <literal>wheel</literal> and <literal>adm</literal> system
     groups with a command such as the following:</para>

     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>

     <para>Note that this command will update the ACLs both for
     existing journal files and for future journal files created in the
     <filename>/var/log/journal/</filename> directory.</para>
   </refsect1>

   <refsect1>
     <title>Files</title>

     <variablelist>
       <varlistentry>
         <term><filename>/etc/systemd/journald.conf</filename></term>

         <listitem><para>Configure <command>systemd-journald</command> behavior. See
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
         </para></listitem>
       </varlistentry>

       <varlistentry>
         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
         <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
         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
         or
         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
         with the <literal>.journal</literal> suffix. If the daemon is
         stopped uncleanly, or if the files are found to be corrupted,
         they are renamed using the <literal>.journal~</literal>
         suffix, and <command>systemd-journald</command> starts writing
         to a new file. <filename>/run/</filename> is used when
         <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>

         <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>

         <para><filename>systemd-journald</filename> will automatically remove the oldest
         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
         and related settings in
         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
         </para></listitem>
       </varlistentry>

       <varlistentry>
         <term><filename>/dev/kmsg</filename></term>
         <term><filename>/dev/log</filename></term>
         <term><filename>/run/systemd/journal/dev-log</filename></term>
         <term><filename>/run/systemd/journal/socket</filename></term>
         <term><filename>/run/systemd/journal/stdout</filename></term>

         <listitem><para>Sockets and other file node paths that <command>systemd-journald</command> will
         listen on and are visible in the file system. In addition to these,
         <command>systemd-journald</command> can listen for audit events using <citerefentry
         project='man-pages'><refentrytitle>netlink</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
       </varlistentry>
     </variablelist>

     <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
   </refsect1>

   <refsect1>
     <title>See Also</title>
     <para>
       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
       <citerefentry project='man-pages'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <command>pydoc systemd.journal</command>
     </para>
   </refsect1>

 </refentry>
	<?xml version='1.0'?> <!---nxml--->
	<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
	<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

	<refentry id="systemd-journald.service">

	<refentryinfo>
	<title>systemd-journald.service</title>
	<productname>systemd</productname>
	</refentryinfo>

	<refmeta>
	<refentrytitle>systemd-journald.service</refentrytitle>
	<manvolnum>8</manvolnum>
	</refmeta>

	<refnamediv>
	<refname>systemd-journald.service</refname>
	<refname>systemd-journald.socket</refname>
	<refname>systemd-journald-dev-log.socket</refname>
	<refname>systemd-journald-audit.socket</refname>
	<refname>systemd-journald@.service</refname>
	<refname>systemd-journald@.socket</refname>
	<refname>systemd-journald-varlink@.socket</refname>
	<refname>systemd-journald</refname>
	<refpurpose>Journal service</refpurpose>
	</refnamediv>

	<refsynopsisdiv>
	<para><filename>systemd-journald.service</filename></para>
	<para><filename>systemd-journald.socket</filename></para>
	<para><filename>systemd-journald-dev-log.socket</filename></para>
	<para><filename>systemd-journald-audit.socket</filename></para>
	<para><filename>systemd-journald@.service</filename></para>
	<para><filename>systemd-journald@.socket</filename></para>
	<para><filename>systemd-journald-varlink@.socket</filename></para>
	<para><filename>/usr/lib/systemd/systemd-journald</filename></para>
	</refsynopsisdiv>

	<refsect1>
	<title>Description</title>

	<para><filename>systemd-journald</filename> is a system service
	that collects and stores logging data. It creates and maintains
	structured, indexed journals based on logging information that is
	received from a variety of sources:</para>

	<itemizedlist>
	<listitem><para>Kernel log messages, via kmsg</para></listitem>

	<listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
	project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
	call</para></listitem>

	<listitem><para>Structured system log messages via the native Journal API, see
	<citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>
	and <ulink url="https://systemd.io/JOURNAL_NATIVE_PROTOCOL">Native Journal
	Protocol</ulink></para></listitem>

	<listitem><para>Standard output and standard error of service units. For further details see
	below.</para></listitem>

	<listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
	</itemizedlist>

	<para>The daemon will implicitly collect numerous metadata fields
	for each log messages in a secure and unfakeable way. See
	<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
	for more information about the collected metadata.
	</para>

	<para>Log data collected by the journal is primarily text-based but can also include binary data where
	necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>

	<para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
	volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
	data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
	to volatile storage otherwise. Use <varname>Storage=</varname> in
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
	where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>

	<para>Note that journald will initially use volatile storage, until a call to
	<command>journalctl --flush</command> (or sending <constant>SIGUSR1</constant> to journald) will cause
	it to switch to persistent logging (under the conditions mentioned above). This is done automatically
	on boot via <literal>systemd-journal-flush.service</literal>.</para>

	<para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
	desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
	ensure it has the correct access modes and ownership:</para>

	<programlisting>mkdir -p /var/log/journal
	systemd-tmpfiles --create --prefix /var/log/journal</programlisting>

	<para>See
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
	for information about the configuration of this service.</para>
	</refsect1>

	<refsect1>
	<title>Stream logging</title>

	<para>The systemd service manager invokes all service processes with standard output and standard error connected
	to the journal by default. This behaviour may be altered via the
	<varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
	<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
	journal converts the log byte stream received this way into individual log records, splitting the stream at newline
	(<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>

	<para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
	services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
	errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
	ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
	write attempts will also result in such process signals being generated, see
	<citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
	To mitigate this issue, systemd service manager explicitly turns off the <constant>SIGPIPE</constant>
	signal for all invoked processes by default (this may be changed for each unit individually via the
	<varname>IgnoreSIGPIPE=</varname> option, see
	<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
	details). After the standard output/standard error streams have been terminated they may not be recovered
	until the services they are associated with are restarted. Note that during normal operation,
	<filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in
	the service manager. If <filename>systemd-journald.service</filename> is restarted using
	<command>systemctl restart</command> or equivalent operation instead of a pair of separate
	<command>systemctl stop</command> and <command>systemctl start</command> commands (or equivalent
	operations), these stream connections are not terminated and survive the restart. It is thus safe to
	restart <filename>systemd-journald.service</filename>, but stopping it is not recommended.</para>

	<para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
	metadata of the peer the stream was originally created for. If the stream connection is passed on to other
	processes (such as further child processes forked off the main service process), the log records will not reflect
	their metadata, but will continue to describe the original process. This is different from the other logging
	transports listed above, which are inherently record based and where the metadata is always associated with the
	individual record.</para>

	<para>In addition to the implicit standard output/error logging of services, stream logging is also available
	via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
	line tool.</para>

	<para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
	4096. When this limit is reached further log streams may be established but will receive
	<constant>EPIPE</constant> right from the beginning.</para>
	</refsect1>

	<refsect1>
	<title>Journal Namespaces</title>

	<para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
	consisting of one or more services from the rest of the system and a mechanism for improving
	performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
	stream managed by its own instance of <command>systemd-journald</command>. Namespaces are independent of
	each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
	exists, managed by <filename>systemd-journald.service</filename> (and its associated socket
	units). Additional namespaces are created by starting an instance of the
	<filename>systemd-journald@.service</filename> service template. The instance name is the namespace
	identifier, which is a short string used for referencing the journal namespace. Service units may be
	assigned to a specific journal namespace through the <varname>LogNamespace=</varname> unit file setting,
	see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
	details. The <option>--namespace=</option> switch of
	<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
	used to view the log stream of a specific namespace. If the switch is not used the log stream of the
	default namespace is shown, i.e. log data from other namespaces is not visible.</para>

	<para>Services associated with a specific log namespace may log via syslog, the native logging protocol
	of the journal and via stdout/stderr; the logging from all three transports is associated with the
	namespace.</para>

	<para>By default only the default namespace will collect kernel and audit log messages.</para>

	<para>The <command>systemd-journald</command> instance of the default namespace is configured through
	<filename>/etc/systemd/journald.conf</filename> (see below), while the other instances are configured
	through <filename>/etc/systemd/journald@<replaceable>NAMESPACE</replaceable>.conf</filename>. The journal
	log data for the default namespace is placed in
	<filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable></filename> (see below) while the data
	for the other namespaces is located in
	<filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable>.<replaceable>NAMESPACE</replaceable></filename>.</para>
	</refsect1>

	<refsect1>
	<title>Signals</title>

	<variablelist>
	<varlistentry>
	<term>SIGUSR1</term>

	<listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
	<filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
	after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run/</filename> is
	never flushed to <filename>/var/</filename> regardless of the configuration. Use the
	<command>journalctl --flush</command> command to request flushing of the journal files, and wait for
	the operation to complete. See
	<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
	details.</para></listitem>
	</varlistentry>

	<varlistentry>
	<term>SIGUSR2</term>

	<listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
	--rotate</command> command to request journal file rotation, and wait for the operation to
	complete.</para></listitem>
	</varlistentry>

	<varlistentry>
	<term>SIGRTMIN+1</term>

	<listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
	--sync</command> command to trigger journal synchronization, and wait for the operation to
	complete.</para></listitem>
	</varlistentry>
	</variablelist>
	</refsect1>

	<refsect1>
	<title>Kernel Command Line</title>

	<para>A few configuration parameters from
	<filename>journald.conf</filename> may be overridden on the kernel
	command line:</para>

	<variablelist class='kernel-commandline-options'>
	<varlistentry>
	<term><varname>systemd.journald.forward_to_syslog=</varname></term>
	<term><varname>systemd.journald.forward_to_kmsg=</varname></term>
	<term><varname>systemd.journald.forward_to_console=</varname></term>
	<term><varname>systemd.journald.forward_to_wall=</varname></term>

	<listitem><para>Enables/disables forwarding of collected log
	messages to syslog, the kernel log buffer, the system console
	or wall.
	</para>

	<para>See
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
	for information about these settings.</para>
	</listitem>

	</varlistentry>
	</variablelist>

	<para>Note that these kernel command line options are only honoured by the default namespace, see
	above.</para>
	</refsect1>

	<refsect1>
	<title>Access Control</title>

	<para>Journal files are, by default, owned and readable by the
	<literal>systemd-journal</literal> system group but are not
	writable. Adding a user to this group thus enables them to read
	the journal files.</para>

	<para>By default, each user, with a UID outside the range of system users,
	dynamic service users, and the nobody user, will get their own set of
	journal files in <filename>/var/log/journal/</filename>. See
	<ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
	for more details about UID ranges. These journal
	files will not be owned by the user, however, in order to avoid
	that the user can write to them directly. Instead, file system
	ACLs are used to ensure the user gets read access only.</para>

	<para>Additional users and groups may be granted access to journal
	files via file system access control lists (ACL). Distributions
	and administrators may choose to grant read access to all members
	of the <literal>wheel</literal> and <literal>adm</literal> system
	groups with a command such as the following:</para>

	<programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>

	<para>Note that this command will update the ACLs both for
	existing journal files and for future journal files created in the
	<filename>/var/log/journal/</filename> directory.</para>
	</refsect1>

	<refsect1>
	<title>Files</title>

	<variablelist>
	<varlistentry>
	<term><filename>/etc/systemd/journald.conf</filename></term>

	<listitem><para>Configure <command>systemd-journald</command> behavior. See
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
	</para></listitem>
	</varlistentry>

	<varlistentry>
	<term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
	<term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
	<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
	<filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
	or
	<filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
	with the <literal>.journal</literal> suffix. If the daemon is
	stopped uncleanly, or if the files are found to be corrupted,
	they are renamed using the <literal>.journal~</literal>
	suffix, and <command>systemd-journald</command> starts writing
	to a new file. <filename>/run/</filename> is used when
	<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>

	<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>

	<para><filename>systemd-journald</filename> will automatically remove the oldest
	archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
	and related settings in
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
	</para></listitem>
	</varlistentry>

	<varlistentry>
	<term><filename>/dev/kmsg</filename></term>
	<term><filename>/dev/log</filename></term>
	<term><filename>/run/systemd/journal/dev-log</filename></term>
	<term><filename>/run/systemd/journal/socket</filename></term>
	<term><filename>/run/systemd/journal/stdout</filename></term>

	<listitem><para>Sockets and other file node paths that <command>systemd-journald</command> will
	listen on and are visible in the file system. In addition to these,
	<command>systemd-journald</command> can listen for audit events using <citerefentry
	project='man-pages'><refentrytitle>netlink</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
	</varlistentry>
	</variablelist>

	<para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
	</refsect1>

	<refsect1>
	<title>See Also</title>
	<para>
	<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
	<citerefentry project='man-pages'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
	<citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
	<command>pydoc systemd.journal</command>
	</para>
	</refsect1>

	</refentry>