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5430f7f2bc
We finally got the OK from all contributors with non-trivial commits to relicense systemd from GPL2+ to LGPL2.1+. Some udev bits continue to be GPL2+ for now, but we are looking into relicensing them too, to allow free copy/paste of all code within systemd. The bits that used to be MIT continue to be MIT. The big benefit of the relicensing is that closed source code may now link against libsystemd-login.so and friends.
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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