179 lines
7.2 KiB
XML
179 lines
7.2 KiB
XML
<?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>
|