nixpkgs/nixos/doc/manual/administration/store-corruption.xml

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        xmlns:xlink="http://www.w3.org/1999/xlink"
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        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>
<prompt># </prompt>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>
<prompt># </prompt>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>