man/systemd-readahead-replay.service.xml

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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 disk 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>--max-files=</option></term>
 128
 129                                 <listitem><para>Maximum number of
 130                                 files to read ahead. Only valid
 131                                 for thes <command>collect</command>
 132                                 command.</para></listitem>
 133                         </varlistentry>
 134
 135                         <varlistentry>
 136                                 <term><option>--file-size-max=</option></term>
 137
 138                                 <listitem><para>Maximum size of files
 139                                 (in bytes) to read ahead. Only valid
 140                                 for the <command>collect</command>
 141                                 and <command>replay</command>
 142                                 commands.</para></listitem>
 143                         </varlistentry>
 144
 145                         <varlistentry>
 146                                 <term><option>--timeout=</option></term>
 147
 148                                 <listitem><para>Maximum time (in usec)
 149                                 to to spend collecting data. Only valid
 150                                 for the <command>collect</command>
 151                                 command.</para></listitem>
 152                         </varlistentry>
 153                 </variablelist>
 154
 155         </refsect1>
 156
 157         <refsect1>
 158                 <title>Commands</title>
 159
 160                 <para>The following commands are understood by
 161                 <filename>systemd-readahead</filename>:</para> <variablelist>
 162                         <varlistentry>
 163                                 <term><command>collect
 164                                 [<replaceable>DIRECTORY</replaceable>]</command></term>
 165                                 <listitem>
 166                                         <para>Collect read-ahead data on
 167                                         early boot. When terminating it will
 168                                         write out a pack file to the indicated
 169                                         directory containing the read-ahead
 170                                         data. </para>
 171                                 </listitem>
 172                         </varlistentry>
 173
 174                         <varlistentry>
 175                                 <term><command>replay
 176                                 [<replaceable>DIRECTORY</replaceable>]</command></term>
 177                                 <listitem>
 178                                         <para>Perform read-ahead on the
 179                                         specified directory tree.</para>
 180                                 </listitem>
 181                         </varlistentry>
 182
 183                         <varlistentry>
 184                                 <term><command>analyze
 185                                 [<replaceable>FILE</replaceable>]</command></term>
 186                                 <listitem>
 187                                         <para>Dumps the content of the
 188                                         read-ahead pack file to the
 189                                         terminal. The output lists
 190                                         approximately for each file
 191                                         how much will be read-ahead by
 192                                         the <command>replay</command>
 193                                         command.</para>
 194                                 </listitem>
 195                         </varlistentry>
 196
 197                 </variablelist>
 198         </refsect1>
 199
 200         <refsect1>
 201                 <title>See Also</title>
 202                 <para>
 203                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 204                 </para>
 205         </refsect1>
 206
 207 </refentry>