man/sd_readahead.xml

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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="sd_readahead" conditional='ENABLE_READAHEAD'>

        <refentryinfo>
                <title>sd_readahead</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>sd_readahead</refentrytitle>
                <manvolnum>3</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>sd_readahead</refname>
                <refpurpose>Control ongoing disk boot-time read-ahead operations</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <funcsynopsis>
                        <funcsynopsisinfo>#include "sd-readahead.h"</funcsynopsisinfo>

                        <funcprototype>
                                <funcdef>int <function>sd_readahead</function></funcdef>
                                <paramdef>const char *<parameter>action</parameter></paramdef>
                        </funcprototype>
                </funcsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>
                <para><function>sd_readahead()</function> may be
                called by programs involved with early boot-up to
                control ongoing boot-time disk read-ahead operations. It may be
                used to terminate read-ahead operations in case an
                uncommon disk access pattern is to be expected and
                hence read-ahead replay or collection is unlikely to
                have the desired speed-up effect on the current or
                future boot-ups.</para>

                <para>The <parameter>action</parameter> should be one
                of the following strings:</para>

                <variablelist>
                        <varlistentry>
                                <term>cancel</term>

                                <listitem><para>Terminates read-ahead
                                data collection, and drops all
                                read-ahead data collected during this
                                boot-up.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>done</term>

                                <listitem><para>Terminates read-ahead
                                data collection, but keeps all
                                read-ahead data collected during this
                                boot-up around for use during
                                subsequent boot-ups.</para></listitem>
                        </varlistentry>

                        <varlistentry>
                                <term>noreplay</term>

                                <listitem><para>Terminates read-ahead
                                replay.</para></listitem>
                        </varlistentry>

                </variablelist>

        </refsect1>

        <refsect1>
                <title>Return Value</title>

                <para>On failure, these calls return a negative
                errno-style error code. It is generally recommended to
                ignore the return value of this call.</para>
        </refsect1>

        <refsect1>
                <title>Notes</title>

                <para>This function is provided by the reference
                implementation of APIs for controlling boot-time
                read-ahead and distributed with the systemd
                package. The algorithm it implements is simple, and
                can easily be reimplemented in daemons if it is
                important to support this interface without using the
                reference implementation.</para>

                <para>Internally, this function creates a file in
                <filename>/run/systemd/readahead/</filename> which is
                then used as flag file to notify the read-ahead
                subsystem.</para>

                <para>For details about the algorithm check the
                liberally licensed reference implementation sources:
                <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c"/>
                and <ulink
                url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h"/></para>

                <para><function>sd_readahead()</function> is
                implemented in the reference implementation's drop-in
                <filename>sd-readahead.c</filename> and
                <filename>sd-readahead.h</filename> files. It is
                recommended that applications consuming this API copy
                the implementation into their source tree. For more
                details about the reference implementation see
                <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry></para>

                <para>If -DDISABLE_SYSTEMD is set during compilation
                this function will always return 0 and otherwise
                become a NOP.</para>
        </refsect1>

        <refsect1>
                <title>Examples</title>

                <example>
                        <title>Cancelling all read-ahead operations</title>

                        <para>During boots where SELinux has to
                        relabel the file system hierarchy, it will
                        create a large amount of disk accesses that
                        are not necessary during normal boots. Hence
                        it is a good idea to disable both read-ahead replay and read-ahead collection.
                        </para>

                        <programlisting>sd_readahead("cancel");
sd_readahead("noreplay");</programlisting>
                </example>

        </refsect1>

        <refsect1>
                <title>See Also</title>
                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>