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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="systemd-readahead-replay.service" conditional='ENABLE_READAHEAD'>
<refentryinfo>
<title>systemd-readahead-replay.service</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-readahead-replay.service</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-readahead-replay.service</refname>
<refname>systemd-readahead-collect.service</refname>
<refname>systemd-readahead-done.service</refname>
<refname>systemd-readahead-done.timer</refname>
<refname>systemd-readahead</refname>
<refpurpose>Disk read ahead logic</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>systemd-readahead-replay.service</filename></para>
<para><filename>systemd-readahead-collect.service</filename></para>
<para><filename>systemd-readahead-done.service</filename></para>
<para><filename>systemd-readahead-done.timer</filename></para>
<para><filename>/usr/lib/systemd/systemd-readahead</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><filename>systemd-readahead-collect.service</filename>
is a service that collects disk usage patterns at boot
time. <filename>systemd-readahead-replay.service</filename>
is a service that replays this access data collected
at the subsequent boot. Since disks tend to be
magnitudes slower than RAM this is intended to improve
boot speeds by pre-loading early at boot all data on
disk that is known to be read for the complete boot
process.</para>
<para><filename>systemd-readahead-done.service</filename>
is executed a short while after boot completed and signals
<filename>systemd-readahead-collect.service</filename>
to end data collection. On this signal this service
will then sort the collected disk accesses and store
information about them disk in
<filename>/.readahead</filename>.</para>
<para>Normally, both
<filename>systemd-readahead-collect.service</filename>
and
<filename>systemd-readahead-replay.service</filename>
are activated at boot so that access patterns from the
preceding boot are replayed and new data collected
for the subsequent boot. However, on read-only media
where the collected data cannot be stored it might
be a good idea to disable
<filename>systemd-readahead-collect.service</filename>.</para>
<para>On rotating media, when replaying disk accesses
at early boot
<filename>systemd-readahead-replay.service</filename>
will order read requests by their location on disk. On
non-rotating media, they will be ordered by their
original access timestamp. If the file system supports
it
<filename>systemd-readahead-collect.service</filename>
will also defragment and rearrange files on disk to
optimize subsequent boot times.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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