--- /dev/null
+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+ This file is part of systemd.
+
+ Copyright 2010 Lennart Poettering
+
+ systemd is free software; you can redistribute it and/or modify it
+ under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 2 of the License, or
+ (at your option) any later version.
+
+ systemd is distributed in the hope that it will be useful, but
+ WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="sd_notify">
+
+ <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 read-ahead operations</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcsynopsisinfo>#include "sd-daemon.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 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 new-style daemons 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>/dev/.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/tree/src/sd-daemon.c"/>
+ resp. <ulink
+ url="http://cgit.freedesktop.org/systemd/tree/src/sd-daemon.h"/></para>
+
+ <para><function>sd_readahead()</function> is
+ implemented in the reference implementation's drop-in
+ <filename>sd-daemon.c</filename> and
+ <filename>sd-daemon.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_daemon</refentrytitle><manvolnum>7</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_daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>
~ianmdlvl / elogind.git / blobdiff