man/systemd-readahead-replay.service.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">
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   8   Copyright 2010 Lennart Poettering
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  23
  24 <refentry id="systemd-readahead-replay.service" conditional='ENABLE_READAHEAD'>
  25
  26         <refentryinfo>
  27                 <title>systemd-readahead-replay.service</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>systemd-readahead-replay.service</refentrytitle>
  42                 <manvolnum>8</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>systemd-readahead-replay.service</refname>
  47                 <refname>systemd-readahead-collect.service</refname>
  48                 <refname>systemd-readahead-done.service</refname>
  49                 <refname>systemd-readahead-done.timer</refname>
  50                 <refname>systemd-readahead</refname>
  51                 <refpurpose>Disk read ahead logic</refpurpose>
  52         </refnamediv>
  53
  54         <refsynopsisdiv>
  55                 <para><filename>systemd-readahead-replay.service</filename></para>
  56                 <para><filename>systemd-readahead-collect.service</filename></para>
  57                 <para><filename>systemd-readahead-done.service</filename></para>
  58                 <para><filename>systemd-readahead-done.timer</filename></para>
  59                 <cmdsynopsis>
  60                         <command>/usr/lib/systemd/systemd-readahead/systemd-readahead</command>
  61                         <arg choice="opt" rep="repeat">OPTIONS</arg>
  62                         <arg choice="plain">COMMAND</arg>
  63                         <arg choice="opt">DIRECTORY | FILE</arg>
  64                 </cmdsynopsis>
  65         </refsynopsisdiv>
  66
  67         <refsect1>
  68                 <title>Description</title>
  69
  70                 <para><filename>systemd-readahead-collect.service</filename>
  71                 is a service that collects disk usage patterns at boot
  72                 time. <filename>systemd-readahead-replay.service</filename>
  73                 is a service that replays this access data collected
  74                 at the subsequent boot. Since disks tend to be
  75                 magnitudes slower than RAM, this is intended to improve
  76                 boot speeds by pre-loading early at boot all data on
  77                 disk that is known to be read for the complete boot
  78                 process.</para>
  79
  80                 <para><filename>systemd-readahead-done.service</filename>
  81                 is executed a short while after boot completed and signals
  82                 <filename>systemd-readahead-collect.service</filename>
  83                 to end data collection. On this signal, this service
  84                 will then sort the collected disk accesses and store
  85                 information about them in
  86                 <filename>/.readahead</filename>.</para>
  87
  88                 <para>Normally, both
  89                 <filename>systemd-readahead-collect.service</filename>
  90                 and
  91                 <filename>systemd-readahead-replay.service</filename>
  92                 are activated at boot so that access patterns from the
  93                 preceding boot are replayed and new data collected
  94                 for the subsequent boot. However, on read-only media
  95                 where the collected data cannot be stored, it might
  96                 be a good idea to disable
  97                 <filename>systemd-readahead-collect.service</filename>.</para>
  98
  99                 <para>On rotating media, when replaying disk accesses
 100                 at early boot,
 101                 <filename>systemd-readahead-replay.service</filename>
 102                 will order read requests by their location on disk. On
 103                 non-rotating media, they will be ordered by their
 104                 original access timestamp. If the file system supports
 105                 it,
 106                 <filename>systemd-readahead-collect.service</filename>
 107                 will also defragment and rearrange files on disk to
 108                 optimize subsequent boot times.</para>
 109         </refsect1>
 110
 111         <refsect1>
 112                 <title>Options</title>
 113
 114                 <para><filename>systemd-readahead</filename> understands
 115                 the following options:</para>
 116
 117                 <variablelist>
 118                         <varlistentry>
 119                                 <term><option>-h</option></term>
 120                                 <term><option>--help</option></term>
 121
 122                                 <listitem><para>Prints a short help text
 123                                 and exits.</para></listitem>
 124                         </varlistentry>
 125
 126                         <varlistentry>
 127                                 <term><option>--version</option></term>
 128
 129                                 <listitem><para>Prints a short version
 130                                 string and exits.</para></listitem>
 131                         </varlistentry>
 132
 133                         <varlistentry>
 134                                 <term><option>--max-files=</option></term>
 135
 136                                 <listitem><para>Maximum number of
 137                                 files to read ahead. Only valid
 138                                 for thes <command>collect</command>
 139                                 command.</para></listitem>
 140                         </varlistentry>
 141
 142                         <varlistentry>
 143                                 <term><option>--file-size-max=</option></term>
 144
 145                                 <listitem><para>Maximum size of files
 146                                 in bytes to read ahead. Only valid
 147                                 for the <command>collect</command>
 148                                 and <command>replay</command>
 149                                 commands.</para></listitem>
 150                         </varlistentry>
 151
 152                         <varlistentry>
 153                                 <term><option>--timeout=</option></term>
 154
 155                                 <listitem><para>Maximum time in microseconds
 156                                 to spend collecting data. Only valid
 157                                 for the <command>collect</command>
 158                                 command.</para></listitem>
 159                         </varlistentry>
 160                 </variablelist>
 161
 162         </refsect1>
 163
 164         <refsect1>
 165                 <title>Commands</title>
 166
 167                 <para>The following commands are understood by
 168                 <filename>systemd-readahead</filename>:</para> <variablelist>
 169                         <varlistentry>
 170                                 <term><command>collect
 171                                 [<replaceable>DIRECTORY</replaceable>]</command></term>
 172                                 <listitem>
 173                                         <para>Collect read-ahead data on
 174                                         early boot. When terminating, it will
 175                                         write out a pack file to the indicated
 176                                         directory containing the read-ahead
 177                                         data. </para>
 178                                 </listitem>
 179                         </varlistentry>
 180
 181                         <varlistentry>
 182                                 <term><command>replay
 183                                 [<replaceable>DIRECTORY</replaceable>]</command></term>
 184                                 <listitem>
 185                                         <para>Perform read-ahead on the
 186                                         specified directory tree.</para>
 187                                 </listitem>
 188                         </varlistentry>
 189
 190                         <varlistentry>
 191                                 <term><command>analyze
 192                                 [<replaceable>FILE</replaceable>]</command></term>
 193                                 <listitem>
 194                                         <para>Dumps the content of the
 195                                         read-ahead pack file to the
 196                                         terminal. For each file, the
 197                                         output lists approximately how
 198                                         much will be read ahead by
 199                                         the <command>replay</command>
 200                                         command.</para>
 201                                 </listitem>
 202                         </varlistentry>
 203
 204                 </variablelist>
 205         </refsect1>
 206
 207         <refsect1>
 208                 <title>See Also</title>
 209                 <para>
 210                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 211                 </para>
 212         </refsect1>
 213
 214 </refentry>