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zsh_completion: Split out zsh _systemd-analyze
[elogind.git] / man / sd_readahead.xml
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 <!--
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8   Copyright 2010 Lennart Poettering
9
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22 -->
23
24 <refentry id="sd_readahead" conditional='ENABLE_READAHEAD'>
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>
47                 <refpurpose>Control ongoing disk boot-time read-ahead operations</refpurpose>
48         </refnamediv>
49
50         <refsynopsisdiv>
51                 <funcsynopsis>
52                         <funcsynopsisinfo>#include "sd-readahead.h"</funcsynopsisinfo>
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
65                 control ongoing boot-time disk read-ahead operations. It may be
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
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>
124
125                 <para>Internally, this function creates a file in
126                 <filename>/run/systemd/readahead/</filename> which is
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:
132                 <ulink url="http://cgit.freedesktop.org/systemd/systemd/plain/src/readahead/sd-readahead.c"/>
133                 and <ulink
134                 url="http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-readahead.h"/></para>
135
136                 <para><function>sd_readahead()</function> is
137                 implemented in the reference implementation's drop-in
138                 <filename>sd-readahead.c</filename> and
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
143                 <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry></para>
144
145                 <para>If -DDISABLE_SYSTEMD is set during compilation
146                 this function will always return 0 and otherwise
147                 become a NOP.</para>
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>,
173                         <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
174                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
175                 </para>
176         </refsect1>
177
178 </refentry>