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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 boot-time 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 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>/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-readahead.c"/>
resp. <ulink
url="http://cgit.freedesktop.org/systemd/tree/src/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>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-readahead</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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