<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-nix-store-corruption">
  <title>Nix Store Corruption</title>
  <para>
    After a system crash, it’s possible for files in the Nix store to
    become corrupted. (For instance, the Ext4 file system has the
    tendency to replace un-synced files with zero bytes.) NixOS tries
    hard to prevent this from happening: it performs a
    <literal>sync</literal> before switching to a new configuration, and
    Nix’s database is fully transactional. If corruption still occurs,
    you may be able to fix it automatically.
  </para>
  <para>
    If the corruption is in a path in the closure of the NixOS system
    configuration, you can fix it by doing
  </para>
  <programlisting>
# nixos-rebuild switch --repair
</programlisting>
  <para>
    This will cause Nix to check every path in the closure, and if its
    cryptographic hash differs from the hash recorded in Nix’s database,
    the path is rebuilt or redownloaded.
  </para>
  <para>
    You can also scan the entire Nix store for corrupt paths:
  </para>
  <programlisting>
# nix-store --verify --check-contents --repair
</programlisting>
  <para>
    Any corrupt paths will be redownloaded if they’re available in a
    binary cache; otherwise, they cannot be repaired.
  </para>
</section>
