+++ /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 Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 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
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
-
-<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>
~ianmdlvl / elogind.git / blobdiff