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

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<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">
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<title>Nix Store Corruption</title>
<para>
After a system crash, its 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 Nixs 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>
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This will cause Nix to check every path in the closure, and if its
cryptographic hash differs from the hash recorded in Nixs database, the
path is rebuilt or redownloaded.
</para>
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<para>
You can also scan the entire Nix store for corrupt paths:
<screen>
# nix-store --verify --check-contents --repair
</screen>
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Any corrupt paths will be redownloaded if theyre available in a binary
cache; otherwise, they cannot be repaired.
</para>
</section>