]>
Commit | Line | Data |
---|---|---|
6624768c LP |
1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | ||
5 | <!-- | |
6 | This file is part of systemd. | |
7 | ||
8 | Copyright 2010 Lennart Poettering | |
9 | ||
10 | systemd is free software; you can redistribute it and/or modify it | |
5430f7f2 LP |
11 | under the terms of the GNU Lesser General Public License as published by |
12 | the Free Software Foundation; either version 2.1 of the License, or | |
6624768c LP |
13 | (at your option) any later version. |
14 | ||
15 | systemd is distributed in the hope that it will be useful, but | |
16 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
5430f7f2 | 18 | Lesser General Public License for more details. |
6624768c | 19 | |
5430f7f2 | 20 | You should have received a copy of the GNU Lesser General Public License |
6624768c LP |
21 | along with systemd; If not, see <http://www.gnu.org/licenses/>. |
22 | --> | |
23 | ||
56ba3c78 | 24 | <refentry id="sd_notify" conditional='ENABLE_READAHEAD'> |
6624768c LP |
25 | |
26 | <refentryinfo> | |
27 | <title>sd_readahead</title> | |
28 | <productname>systemd</productname> | |
29 | ||
30 | <authorgroup> | |
31 | <author> | |
32 | <contrib>Developer</contrib> | |
33 | <firstname>Lennart</firstname> | |
34 | <surname>Poettering</surname> | |
35 | <email>lennart@poettering.net</email> | |
36 | </author> | |
37 | </authorgroup> | |
38 | </refentryinfo> | |
39 | ||
40 | <refmeta> | |
41 | <refentrytitle>sd_readahead</refentrytitle> | |
42 | <manvolnum>3</manvolnum> | |
43 | </refmeta> | |
44 | ||
45 | <refnamediv> | |
46 | <refname>sd_readahead</refname> | |
e62e6670 | 47 | <refpurpose>Control ongoing disk boot-time read-ahead operations</refpurpose> |
6624768c LP |
48 | </refnamediv> |
49 | ||
50 | <refsynopsisdiv> | |
51 | <funcsynopsis> | |
559de128 | 52 | <funcsynopsisinfo>#include "sd-readahead.h"</funcsynopsisinfo> |
6624768c LP |
53 | |
54 | <funcprototype> | |
55 | <funcdef>int <function>sd_readahead</function></funcdef> | |
56 | <paramdef>const char *<parameter>action</parameter></paramdef> | |
57 | </funcprototype> | |
58 | </funcsynopsis> | |
59 | </refsynopsisdiv> | |
60 | ||
61 | <refsect1> | |
62 | <title>Description</title> | |
63 | <para><function>sd_readahead()</function> may be | |
64 | called by programs involved with early boot-up to | |
e62e6670 | 65 | control ongoing boot-time disk read-ahead operations. It may be |
6624768c LP |
66 | used to terminate read-ahead operations in case an |
67 | uncommon disk access pattern is to be expected and | |
68 | hence read-ahead replay or collection is unlikely to | |
69 | have the desired speed-up effect on the current or | |
70 | future boot-ups.</para> | |
71 | ||
72 | <para>The <parameter>action</parameter> should be one | |
73 | of the following strings:</para> | |
74 | ||
75 | <variablelist> | |
76 | <varlistentry> | |
77 | <term>cancel</term> | |
78 | ||
79 | <listitem><para>Terminates read-ahead | |
80 | data collection, and drops all | |
81 | read-ahead data collected during this | |
82 | boot-up.</para></listitem> | |
83 | </varlistentry> | |
84 | ||
85 | <varlistentry> | |
86 | <term>done</term> | |
87 | ||
88 | <listitem><para>Terminates read-ahead | |
89 | data collection, but keeps all | |
90 | read-ahead data collected during this | |
91 | boot-up around for use during | |
92 | subsequent boot-ups.</para></listitem> | |
93 | </varlistentry> | |
94 | ||
95 | <varlistentry> | |
96 | <term>noreplay</term> | |
97 | ||
98 | <listitem><para>Terminates read-ahead | |
99 | replay.</para></listitem> | |
100 | </varlistentry> | |
101 | ||
102 | </variablelist> | |
103 | ||
104 | </refsect1> | |
105 | ||
106 | <refsect1> | |
107 | <title>Return Value</title> | |
108 | ||
109 | <para>On failure, these calls return a negative | |
110 | errno-style error code. It is generally recommended to | |
111 | ignore the return value of this call.</para> | |
112 | </refsect1> | |
113 | ||
114 | <refsect1> | |
115 | <title>Notes</title> | |
116 | ||
117 | <para>This function is provided by the reference | |
e62e6670 LP |
118 | implementation of APIs for controlling boot-time |
119 | read-ahead and distributed with the systemd | |
120 | package. The algorithm it implements is simple, and | |
121 | can easily be reimplemented in daemons if it is | |
122 | important to support this interface without using the | |
123 | reference implementation.</para> | |
6624768c LP |
124 | |
125 | <para>Internally, this function creates a file in | |
2b583ce6 | 126 | <filename>/run/systemd/readahead/</filename> which is |
6624768c LP |
127 | then used as flag file to notify the read-ahead |
128 | subsystem.</para> | |
129 | ||
130 | <para>For details about the algorithm check the | |
131 | liberally licensed reference implementation sources: | |
a26c9cc6 | 132 | <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c"/> |
16dad32e | 133 | and <ulink |
a26c9cc6 | 134 | url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h"/></para> |
6624768c LP |
135 | |
136 | <para><function>sd_readahead()</function> is | |
559de128 | 137 | implemented in the reference implementation's drop-in |
e62e6670 | 138 | <filename>sd-readahead.c</filename> and |
559de128 LP |
139 | <filename>sd-readahead.h</filename> files. It is |
140 | recommended that applications consuming this API copy | |
141 | the implementation into their source tree. For more | |
142 | details about the reference implementation see | |
cb07866b | 143 | <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry></para> |
559de128 LP |
144 | |
145 | <para>If -DDISABLE_SYSTEMD is set during compilation | |
146 | this function will always return 0 and otherwise | |
147 | become a NOP.</para> | |
6624768c LP |
148 | </refsect1> |
149 | ||
150 | <refsect1> | |
151 | <title>Examples</title> | |
152 | ||
153 | <example> | |
154 | <title>Cancelling all read-ahead operations</title> | |
155 | ||
156 | <para>During boots where SELinux has to | |
157 | relabel the file system hierarchy, it will | |
158 | create a large amount of disk accesses that | |
159 | are not necessary during normal boots. Hence | |
160 | it is a good idea to disable both read-ahead replay and read-ahead collection. | |
161 | </para> | |
162 | ||
163 | <programlisting>sd_readahead("cancel"); | |
164 | sd_readahead("noreplay");</programlisting> | |
165 | </example> | |
166 | ||
167 | </refsect1> | |
168 | ||
169 | <refsect1> | |
170 | <title>See Also</title> | |
171 | <para> | |
172 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
cb07866b | 173 | <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
6624768c LP |
174 | <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry> |
175 | </para> | |
176 | </refsect1> | |
177 | ||
178 | </refentry> |