readahead: add interface to sd-daemon.[ch] to control readahead

1 files changed, 178 insertions, 0 deletions

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+<?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>