<section xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        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
<command>sync</command> 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

<screen>
# nixos-rebuild switch --repair
</screen>

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:

<screen>
# nix-store --verify --check-contents --repair
</screen>

Any corrupt paths will be redownloaded if they’re available in a
binary cache; otherwise, they cannot be repaired.</para>

</section>
