199 lines
6.7 KiB
XML
199 lines
6.7 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>Troubleshooting</title>
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<!--===============================================================-->
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<section><title>Boot problems</title>
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<para>If NixOS fails to boot, there are a number of kernel command
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line parameters that may help you to identify or fix the issue. You
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can add these parameters in the GRUB boot menu by pressing “e” to
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modify the selected boot entry and editing the line starting with
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<literal>linux</literal>. The following are some useful kernel command
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line parameters that are recognised by the NixOS boot scripts or by
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systemd:
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<variablelist>
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<varlistentry><term><literal>boot.shell_on_fail</literal></term>
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<listitem><para>Start a root shell if something goes wrong in
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stage 1 of the boot process (the initial ramdisk). This is
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disabled by default because there is no authentication for the
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root shell.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>boot.debug1</literal></term>
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<listitem><para>Start an interactive shell in stage 1 before
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anything useful has been done. That is, no modules have been
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loaded and no file systems have been mounted, except for
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<filename>/proc</filename> and
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<filename>/sys</filename>.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>boot.trace</literal></term>
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<listitem><para>Print every shell command executed by the stage 1
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and 2 boot scripts.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>single</literal></term>
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<listitem><para>Boot into rescue mode (a.k.a. single user mode).
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This will cause systemd to start nothing but the unit
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<literal>rescue.target</literal>, which runs
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<command>sulogin</command> to prompt for the root password and
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start a root login shell. Exiting the shell causes the system to
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continue with the normal boot process.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>systemd.log_level=debug systemd.log_target=console</literal></term>
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<listitem><para>Make systemd very verbose and send log messages to
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the console instead of the journal.</para></listitem>
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</varlistentry>
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</variablelist>
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For more parameters recognised by systemd, see
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
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<para>If no login prompts or X11 login screens appear (e.g. due to
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hanging dependencies), you can press Alt+ArrowUp. If you’re lucky,
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this will start rescue mode (described above). (Also note that since
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most units have a 90-second timeout before systemd gives up on them,
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the <command>agetty</command> login prompts should appear eventually
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unless something is very wrong.)</para>
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</section>
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<!--===============================================================-->
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<section><title>Maintenance mode</title>
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<para>You can enter rescue mode by running:
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<screen>
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$ systemctl rescue</screen>
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This will eventually give you a single-user root shell. Systemd will
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stop (almost) all system services. To get out of maintenance mode,
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just exit from the rescue shell.</para>
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</section>
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<!--===============================================================-->
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<section><title>Rolling back configuration changes</title>
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<para>After running <command>nixos-rebuild</command> to switch to a
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new configuration, you may find that the new configuration doesn’t
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work very well. In that case, there are several ways to return to a
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previous configuration.</para>
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<para>First, the GRUB boot manager allows you to boot into any
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previous configuration that hasn’t been garbage-collected. These
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configurations can be found under the GRUB submenu “NixOS - All
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configurations”. This is especially useful if the new configuration
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fails to boot. After the system has booted, you can make the selected
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configuration the default for subsequent boots:
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<screen>
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$ /run/current-system/bin/switch-to-configuration boot</screen>
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</para>
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<para>Second, you can switch to the previous configuration in a running
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system:
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<screen>
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$ nixos-rebuild switch --rollback</screen>
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This is equivalent to running:
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<screen>
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$ /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>
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where <replaceable>N</replaceable> is the number of the NixOS system
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configuration. To get a list of the available configurations, do:
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<screen>
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$ ls -l /nix/var/nix/profiles/system-*-link
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<replaceable>...</replaceable>
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lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
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</screen>
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</para>
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</section>
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<!--===============================================================-->
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<section><title>Nix store corruption</title>
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<para>After a system crash, it’s possible for files in the Nix store
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to become corrupted. (For instance, the Ext4 file system has the
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tendency to replace un-synced files with zero bytes.) NixOS tries
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hard to prevent this from happening: it performs a
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<command>sync</command> before switching to a new configuration, and
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Nix’s database is fully transactional. If corruption still occurs,
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you may be able to fix it automatically.</para>
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<para>If the corruption is in a path in the closure of the NixOS
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system configuration, you can fix it by doing
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<screen>
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$ nixos-rebuild switch --repair
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</screen>
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This will cause Nix to check every path in the closure, and if its
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cryptographic hash differs from the hash recorded in Nix’s database,
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the path is rebuilt or redownloaded.</para>
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<para>You can also scan the entire Nix store for corrupt paths:
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<screen>
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$ nix-store --verify --check-contents --repair
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</screen>
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Any corrupt paths will be redownloaded if they’re available in a
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binary cache; otherwise, they cannot be repaired.</para>
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</section>
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<!--===============================================================-->
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<section><title>Nix network issues</title>
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<para>Nix uses a so-called <emphasis>binary cache</emphasis> to
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optimise building a package from source into downloading it as a
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pre-built binary. That is, whenever a command like
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<command>nixos-rebuild</command> needs a path in the Nix store, Nix
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will try to download that path from the Internet rather than build it
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from source. The default binary cache is
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<uri>http://cache.nixos.org/</uri>. If this cache is unreachable, Nix
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operations may take a long time due to HTTP connection timeouts. You
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can disable the use of the binary cache by adding <option>--option
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use-binary-caches false</option>, e.g.
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<screen>
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$ nixos-rebuild switch --option use-binary-caches false
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</screen>
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If you have an alternative binary cache at your disposal, you can use
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it instead:
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<screen>
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$ nixos-rebuild switch --option binary-caches http://my-cache.example.org/
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</screen>
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</para>
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</section>
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</chapter>
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