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man: document that sd-daemon.[ch] is now available as shared library
[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
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23
24 <refentry id="sd_notify">
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-daemon.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/tree/src/sd-readahead.c"/>
133                 resp. <ulink
134                 url="http://cgit.freedesktop.org/systemd/tree/src/sd-readahead.h"/></para>
135
136                 <para><function>sd_readahead()</function> is
137                 implemented in the reference implementation's
138                 <filename>sd-readahead.c</filename> and
139                 <filename>sd-readahead.h</filename> files. These
140                 interfaces are available as shared library, which can
141                 be compiled and linked to with the
142                 <literal>libsystemd-daemon</literal>
143                 <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
144                 file. Alternatively, applications consuming this API
145                 may copy the implementation into their source
146                 tree. For more details about the reference
147                 implementation see
148                 <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
149
150                 <para>If the reference implementation is used as
151                 drop-in files and -DDISABLE_SYSTEMD is set during
152                 compilation this function will always return 0 and
153                 otherwise become a NOP.</para>
154         </refsect1>
155
156         <refsect1>
157                 <title>Examples</title>
158
159                 <example>
160                         <title>Cancelling all read-ahead operations</title>
161
162                         <para>During boots where SELinux has to
163                         relabel the file system hierarchy, it will
164                         create a large amount of disk accesses that
165                         are not necessary during normal boots. Hence
166                         it is a good idea to disable both read-ahead replay and read-ahead collection.
167                         </para>
168
169                         <programlisting>sd_readahead("cancel");
170 sd_readahead("noreplay");</programlisting>
171                 </example>
172
173         </refsect1>
174
175         <refsect1>
176                 <title>See Also</title>
177                 <para>
178                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
179                         <citerefentry><refentrytitle>sd-readahead</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
180                         <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>
181                 </para>
182         </refsect1>
183
184 </refentry>