179 lines
7.3 KiB
XML
179 lines
7.3 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="sd_notify">
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<refentryinfo>
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<title>sd_readahead</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_readahead</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_readahead</refname>
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<refpurpose>Control ongoing disk boot-time read-ahead operations</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include "sd-readahead.h"</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_readahead</function></funcdef>
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<paramdef>const char *<parameter>action</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_readahead()</function> may be
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called by programs involved with early boot-up to
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control ongoing boot-time disk read-ahead operations. It may be
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used to terminate read-ahead operations in case an
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uncommon disk access pattern is to be expected and
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hence read-ahead replay or collection is unlikely to
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have the desired speed-up effect on the current or
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future boot-ups.</para>
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<para>The <parameter>action</parameter> should be one
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of the following strings:</para>
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<variablelist>
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<varlistentry>
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<term>cancel</term>
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<listitem><para>Terminates read-ahead
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data collection, and drops all
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read-ahead data collected during this
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boot-up.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>done</term>
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<listitem><para>Terminates read-ahead
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data collection, but keeps all
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read-ahead data collected during this
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boot-up around for use during
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subsequent boot-ups.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>noreplay</term>
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<listitem><para>Terminates read-ahead
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replay.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On failure, these calls return a negative
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errno-style error code. It is generally recommended to
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ignore the return value of this call.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>This function is provided by the reference
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implementation of APIs for controlling boot-time
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read-ahead and distributed with the systemd
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package. The algorithm it implements is simple, and
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can easily be reimplemented in daemons if it is
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important to support this interface without using the
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reference implementation.</para>
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<para>Internally, this function creates a file in
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<filename>/run/systemd/readahead/</filename> which is
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then used as flag file to notify the read-ahead
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subsystem.</para>
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<para>For details about the algorithm check the
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liberally licensed reference implementation sources:
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<ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c"/>
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resp. <ulink
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url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h"/></para>
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<para><function>sd_readahead()</function> is
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implemented in the reference implementation's drop-in
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<filename>sd-readahead.c</filename> and
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<filename>sd-readahead.h</filename> files. It is
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recommended that applications consuming this API copy
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the implementation into their source tree. For more
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details about the reference implementation see
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<citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>7</manvolnum></citerefentry></para>
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<para>If -DDISABLE_SYSTEMD is set during compilation
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this function will always return 0 and otherwise
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become a NOP.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<example>
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<title>Cancelling all read-ahead operations</title>
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<para>During boots where SELinux has to
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relabel the file system hierarchy, it will
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create a large amount of disk accesses that
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are not necessary during normal boots. Hence
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it is a good idea to disable both read-ahead replay and read-ahead collection.
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</para>
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<programlisting>sd_readahead("cancel");
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sd_readahead("noreplay");</programlisting>
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</example>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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