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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

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<refentry id="systemd-readahead-replay.service" conditional='ENABLE_READAHEAD'
          xmlns:xi="http://www.w3.org/2001/XInclude">

        <refentryinfo>
                <title>systemd-readahead-replay.service</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>systemd-readahead-replay.service</refentrytitle>
                <manvolnum>8</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>systemd-readahead-replay.service</refname>
                <refname>systemd-readahead-collect.service</refname>
                <refname>systemd-readahead-done.service</refname>
                <refname>systemd-readahead-done.timer</refname>
                <refname>systemd-readahead</refname>
                <refpurpose>Disk read ahead logic</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <para><filename>systemd-readahead-replay.service</filename></para>
                <para><filename>systemd-readahead-collect.service</filename></para>
                <para><filename>systemd-readahead-done.service</filename></para>
                <para><filename>systemd-readahead-done.timer</filename></para>
                <cmdsynopsis>
                        <command>/usr/lib/systemd/systemd-readahead/systemd-readahead</command>
                        <arg choice="opt" rep="repeat">OPTIONS</arg>
                        <arg choice="plain">COMMAND</arg>
                        <arg choice="opt">DIRECTORY | FILE</arg>
                </cmdsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para><filename>systemd-readahead-collect.service</filename>
                is a service that collects disk usage patterns at boot
                time. <filename>systemd-readahead-replay.service</filename>
                is a service that replays this access data collected
                at the subsequent boot. Since disks tend to be
                magnitudes slower than RAM, this is intended to improve
                boot speeds by pre-loading early at boot all data on
                disk that is known to be read for the complete boot
                process.</para>

                <para><filename>systemd-readahead-done.service</filename>
                is executed a short while after boot completed and signals
                <filename>systemd-readahead-collect.service</filename>
                to end data collection. On this signal, this service
                will then sort the collected disk accesses and store
                information about them in
                <filename>/.readahead</filename>.</para>

                <para>Normally, both
                <filename>systemd-readahead-collect.service</filename>
                and
                <filename>systemd-readahead-replay.service</filename>
                are activated at boot so that access patterns from the
                preceding boot are replayed and new data collected
                for the subsequent boot. However, on read-only media
                where the collected data cannot be stored, it might
                be a good idea to disable
                <filename>systemd-readahead-collect.service</filename>.</para>

                <para>On rotating media, when replaying disk accesses
                at early boot,
                <filename>systemd-readahead-replay.service</filename>
                will order read requests by their location on disk. On
                non-rotating media, they will be ordered by their
                original access timestamp. If the file system supports
                it,
                <filename>systemd-readahead-collect.service</filename>
                will also defragment and rearrange files on disk to
                optimize subsequent boot times.</para>
        </refsect1>

        <refsect1>
                <title>Options</title>

                <para><filename>systemd-readahead</filename> understands
                the following options:</para>

                <variablelist>
                        <varlistentry>
                                <term><option>--max-files=</option></term>

                                <listitem><para>Maximum number of
                                files to read ahead. Only valid
                                for thes <command>collect</command>
                                command.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><option>--file-size-max=</option></term>

                                <listitem><para>Maximum size of files
                                in bytes to read ahead. Only valid
                                for the <command>collect</command>
                                and <command>replay</command>
                                commands.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><option>--timeout=</option></term>

                                <listitem><para>Maximum time in microseconds
                                to spend collecting data. Only valid
                                for the <command>collect</command>
                                command.</para></listitem>
                        </varlistentry>

                        <xi:include href="standard-options.xml" xpointer="help" />
                        <xi:include href="standard-options.xml" xpointer="version" />
                </variablelist>

        </refsect1>

        <refsect1>
                <title>Commands</title>

                <para>The following commands are understood by
                <filename>systemd-readahead</filename>:</para> <variablelist>
                        <varlistentry>
                                <term><command>collect
                                [<replaceable>DIRECTORY</replaceable>]</command></term>
                                <listitem>
                                        <para>Collect read-ahead data on
                                        early boot. When terminating, it will
                                        write out a pack file to the indicated
                                        directory containing the read-ahead
                                        data. </para>
                                </listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><command>replay
                                [<replaceable>DIRECTORY</replaceable>]</command></term>
                                <listitem>
                                        <para>Perform read-ahead on the
                                        specified directory tree.</para>
                                </listitem>
                        </varlistentry>

                        <varlistentry>
                                <term><command>analyze
                                [<replaceable>FILE</replaceable>]</command></term>
                                <listitem>
                                        <para>Dumps the content of the
                                        read-ahead pack file to the
                                        terminal. For each file, the
                                        output lists approximately how
                                        much will be read ahead by
                                        the <command>replay</command>
                                        command.</para>
                                </listitem>
                        </varlistentry>

                </variablelist>
        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>