man: document the readahead services

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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">
+
+        <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>systemd 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
+                preceeding 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>