From a099ca45054940b63b1615920de158ebafb25ea8 Mon Sep 17 00:00:00 2001
From: Mikey Ariel <mariel@redhat.com>
Date: Sun, 24 Aug 2014 19:18:18 +0200
Subject: Chunk NixOS manual

[Squashed commits to make git blame etc. more likely to work. -ED]
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 nixos/doc/manual/installation/upgrading.xml | 90 +++++++++++++++++++++++++++++
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+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         version="5.0"
+         xml:id="sec-upgrading">
+
+<title>Upgrading NixOS</title>
+
+<para>The best way to keep your NixOS installation up to date is to
+use one of the NixOS <emphasis>channels</emphasis>.  A channel is a
+Nix mechanism for distributing Nix expressions and associated
+binaries.  The NixOS channels are updated automatically from NixOS’s
+Git repository after certain tests have passed and all packages have
+been built.  These channels are:
+
+<itemizedlist>
+  <listitem>
+    <para>Stable channels, such as <literal
+    xlink:href="http://nixos.org/channels/nixos-14.04">nixos-14.04</literal>.
+    These only get conservative bug fixes and package upgrades.  For
+    instance, a channel update may cause the Linux kernel on your
+    system to be upgraded from 3.4.66 to 3.4.67 (a minor bug fix), but
+    not from 3.4.<replaceable>x</replaceable> to
+    3.11.<replaceable>x</replaceable> (a major change that has the
+    potential to break things).  Stable channels are generally
+    maintained until the next stable branch is created.</para>
+  </listitem>
+  <listitem>
+    <para>The unstable channel, <literal
+    xlink:href="http://nixos.org/channels/nixos-unstable">nixos-unstable</literal>.
+    This corresponds to NixOS’s main development branch, and may thus
+    see radical changes between channel updates.  It’s not recommended
+    for production systems.</para>
+  </listitem>
+</itemizedlist>
+
+To see what channels are available, go to <link
+xlink:href="http://nixos.org/channels"/>.  (Note that the URIs of the
+various channels redirect to a directory that contains the channel’s
+latest version and includes ISO images and VirtualBox
+appliances.)</para>
+
+<para>When you first install NixOS, you’re automatically subscribed to
+the NixOS channel that corresponds to your installation source.   For
+instance, if you installed from a 14.04 ISO, you will be subscribed to
+the <literal>nixos-14.04</literal> channel.  To see which NixOS
+channel you’re subscribed to, run the following as root:
+
+<screen>
+$ nix-channel --list | grep nixos
+nixos https://nixos.org/channels/nixos-unstable
+</screen>
+
+To switch to a different NixOS channel, do
+
+<screen>
+$ nix-channel --add http://nixos.org/channels/<replaceable>channel-name</replaceable> nixos
+</screen>
+
+(Be sure to include the <literal>nixos</literal> parameter at the
+end.)  For instance, to use the NixOS 14.04 stable channel:
+
+<screen>
+$ nix-channel --add http://nixos.org/channels/nixos-14.04 nixos
+</screen>
+
+But it you want to live on the bleeding edge:
+
+<screen>
+$ nix-channel --add http://nixos.org/channels/nixos-unstable nixos
+</screen>
+
+</para>
+
+<para>You can then upgrade NixOS to the latest version in your chosen
+channel by running
+
+<screen>
+$ nixos-rebuild switch --upgrade
+</screen>
+
+which is equivalent to the more verbose <literal>nix-channel --update
+nixos; nixos-rebuild switch</literal>.</para>
+
+<warning><para>It is generally safe to switch back and forth between
+channels.  The only exception is that a newer NixOS may also have a
+newer Nix version, which may involve an upgrade of Nix’s database
+schema.  This cannot be undone easily, so in that case you will not be
+able to go back to your original channel.</para></warning>
+
+</chapter>
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