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">
  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                 <para><filename>/usr/lib/systemd/systemd-readahead</filename></para>
  60         </refsynopsisdiv>
  61
  62         <refsect1>
  63                 <title>Description</title>
  64
  65                 <para><filename>systemd-readahead-collect.service</filename>
  66                 is a service that collects disk usage patterns at boot
  67                 time. <filename>systemd-readahead-replay.service</filename>
  68                 is a service that replays this access data collected
  69                 at the subsequent boot. Since disks tend to be
  70                 magnitudes slower than RAM this is intended to improve
  71                 boot speeds by pre-loading early at boot all data on
  72                 disk that is known to be read for the complete boot
  73                 process.</para>
  74
  75                 <para><filename>systemd-readahead-done.service</filename>
  76                 is executed a short while after boot completed and signals
  77                 <filename>systemd-readahead-collect.service</filename>
  78                 to end data collection. On this signal this service
  79                 will then sort the collected disk accesses and store
  80                 information about them disk in
  81                 <filename>/.readahead</filename>.</para>
  82
  83                 <para>Normally, both
  84                 <filename>systemd-readahead-collect.service</filename>
  85                 and
  86                 <filename>systemd-readahead-replay.service</filename>
  87                 are activated at boot so that access patterns from the
  88                 preceding boot are replayed and new data collected
  89                 for the subsequent boot. However, on read-only media
  90                 where the collected data cannot be stored it might
  91                 be a good idea to disable
  92                 <filename>systemd-readahead-collect.service</filename>.</para>
  93
  94                 <para>On rotating media, when replaying disk accesses
  95                 at early boot
  96                 <filename>systemd-readahead-replay.service</filename>
  97                 will order read requests by their location on disk. On
  98                 non-rotating media, they will be ordered by their
  99                 original access timestamp. If the file system supports
 100                 it
 101                 <filename>systemd-readahead-collect.service</filename>
 102                 will also defragment and rearrange files on disk to
 103                 optimize subsequent boot times.</para>
 104         </refsect1>
 105
 106         <refsect1>
 107                 <title>See Also</title>
 108                 <para>
 109                         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 110                 </para>
 111         </refsect1>
 112
 113 </refentry>