systemd/man/systemd-readahead-replay.service.xml

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<refentry id="systemd-readahead-replay.service" conditional='ENABLE_READAHEAD'>

        <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>
                <para><filename>/usr/lib/systemd/systemd-readahead</filename></para>
        </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 disk 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>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>