ZFS

ZFS (wikipedia:en:ZFS) - also known as OpenZFS (wikipedia:en:OpenZFS) - is modern filesystem which is well supported on NixOS.

There are a lot of packages for ZFS. For example there is the zfs package (ZFS Filesystem Linux Kernel module) itself. But there are also a lot of packages of the ZFS ecosystem available.

ZFS is also modularized very well. For example it is available for boot and as a service.

Limitations

latestCompatibleLinuxPackages of ZFS for boot.kernelPackages

Newest kernels might not be supported by ZFS yet. If you are running an newer kernel which is not yet officially supported by zfs, the zfs module will refuse to evaluate and show up as broken. Use boot.kernelPackages = config.boot.zfs.package.latestCompatibleLinuxPackages; to use the latest compatible kernel.

missing support for SWAP on ZFS

ZFS does not support swapfiles. Hibernation must be either disabled with boot.kernelParams = [ "nohibernate" ];, or be enabled with a separate, non-ZFS swap partition.

boot.zfs.devNodes in virtual machines

If you are running within a VM and NixOS fails to import the zpool on reboot, you may need to add boot.zfs.devNodes = "/dev/disk/by-path"; to your configuration.nix file.

declarative mounting of ZFS datasets

When using legacy mountpoints (created with egzfs create -o mountpoint=legacy) mountpoints must be specified with fileSystems."/mount/point" = {};. ZFS native mountpoints are not managed as part of the system configuration but better support hibernation with a separate swap partition. This can lead to conflicts if ZFS mount service is also enabled for the same datasets. Disable it with systemd.services.zfs-mount.enable = false;.

Guides

OpenZFS Documentation for installing

A guide for a NixOS installation with ZFS is maintained at OpenZFS Documentation (Getting Started for NixOS)

It is about:

• enabling ZFS on an existing NixOS installation and
• (installing NixOS with) Root on ZFS.

It is not about:

• integrating ZFS into your existing config

If an issue arises for this guide, submit an issue or pull request there.

Importing on boot

If you create a zpool, it will not be imported on the next boot unless you either add the zpool name to boot.zfs.extraPools:

## In /etc/nixos/configuration.nix:
boot.zfs.extraPools = [ "zpool_name" ];

or if you are using legacy mountpoints, add a fileSystems entry and NixOS will automatically detect that the pool needs to be imported:

## In /etc/nixos/configuration.nix:
fileSystems."/mount/point" = {
  device = "zpool_name";
  fsType = "zfs";
};

Mount datasets at boot

zfs-mount service is enabled by default on NixOS 22.05.

To automatically mount a dataset at boot, you only need to set canmount=on and mountpoint=/mount/point on the respective datasets.

Changing the Adaptive Replacement Cache size

To change the maximum size of the ARC to (for example) 12 GB, add this to your NixOS configuration:

boot.kernelParams = [ "zfs.zfs_arc_max=12884901888" ];

Tuning other parameters

To tune other attributes of ARC, L2ARC or of ZFS itself via runtime modprobe config, add this to your NixOS configuration (keys and values are examples only!):

    boot.extraModprobeConfig = ''
      options zfs l2arc_noprefetch=0 l2arc_write_boost=33554432 l2arc_write_max=16777216 zfs_arc_max=2147483648
    '';

You can confirm whether any specified configuration/tuning got applied via commands like arc_summary and arcstat -a -s " ".

Automatic scrubbing

Regular scrubbing of ZFS pools is recommended and can be enabled in your NixOS configuration via:

services.zfs.autoScrub.enable = true;

You can tweak the interval (defaults to once a week) and which pools should be scrubbed (defaults to all).

Reservations

On ZFS, the performance will deteriorate significantly when more than 80% of the available space is used. To avoid this, reserve disk space beforehand.

To reserve space create a new unused dataset that gets a guaranteed disk space of 10GB.

# zfs create -o refreservation=10G -o mountpoint=none zroot/reserved

Auto ZFS trimming

services.zfs.trim.enable = true;.

This will periodically run zpool trim. Note that this is different from the autotrim pool property. For further information, see the zpool-trim and zpoolprops man pages.

Take snapshots automatically

See services.sanoid section in man configuration.nix.

NFS share

With sharenfs property, ZFS has build-in support for generating /etc/exports.d/zfs.exports file, which in turn is processed by NFS service automatically.

Warning: If you are intending on defining an IPv6 subnet as part of your sharenfs rule, as of ZFS 2.0.6 (2021-09-23) please note that due to a bug in openzfs your rule will not correctly apply, and may result in a security vulnerability (CVE-2013-20001). A fix has been implemented in the next yet-to-be-released upstream version - openzfs/zfs#11939

To enable NFS share on a dataset, only two steps are needed:

First, enable NFS service:

services.nfs.server.enable = true;

Only this line is needed. Configure firewall if necessary, as described in NFS article.

Then, set sharenfs property:

# zfs set sharenfs="ro=192.168.1.0/24,all_squash,anonuid=70,anongid=70" rpool/myData

For more options, see man 5 exports.

Todo: sharesmb property for Samba.

Mail notification for ZFS Event Daemon

ZFS Event Daemon (zed) monitors events generated by the ZFS kernel module and runs configured tasks. It can be configured to send an email when a pool scrub is finished or a disk has failed. zed options

Alternative 1: Enable Mail Notification without Re-compliation

First, we need to configure a mail transfer agent, the program that sends email:

{
  programs.msmtp = {
    enable = true;
    setSendmail = true;
    defaults = {
      aliases = "/etc/aliases";
      port = 465;
      tls_trust_file = "/etc/ssl/certs/ca-certificates.crt";
      tls = "on";
      auth = "login";
      tls_starttls = "off";
    };
    accounts = {
      default = {
        host = "mail.example.com";
        passwordeval = "cat /etc/emailpass.txt";
        user = "user@example.com";
        from = "user@example.com";
      };
    };
  };
}

Then, configure an alias for root account. With this alias configured, all mails sent to root, such as cron job results and failed sudo login events, will be redirected to the configured email account.

tee -a /etc/aliases <<EOF
root: user@example.com
EOF

Finally, override default zed settings with a custom one:

{
  services.zfs.zed.settings = {
    ZED_DEBUG_LOG = "/tmp/zed.debug.log";
    ZED_EMAIL_ADDR = [ "root" ];
    ZED_EMAIL_PROG = "${pkgs.msmtp}/bin/msmtp";
    ZED_EMAIL_OPTS = "@ADDRESS@";

    ZED_NOTIFY_INTERVAL_SECS = 3600;
    ZED_NOTIFY_VERBOSE = true;

    ZED_USE_ENCLOSURE_LEDS = true;
    ZED_SCRUB_AFTER_RESILVER = true;
  };
  # this option does not work; will return error
  services.zfs.zed.enableMail = false;
}

You can now test this by performing a scrub

# zpool scrub $pool

Alternative 2: Rebuild ZFS with Mail Support

The zfs package can be rebuilt with mail features. However, please note that this will cause Nix to recompile the entire ZFS package on the computer, and on every kernel update, which could be very time-consuming on lower-end NAS systems.

An alternative solution that does not involve recompliation can be found above.

The following override is needed as zfs is implicitly used in partition mounting:

nixpkgs.config.packageOverrides = pkgs: {
  zfsStable = pkgs.zfsStable.override { enableMail = true; };
};

A mail sender like msmtp or postfix is required.

A minimal, testable ZED configuration example:

services.zfs.zed.enableMail = true;
services.zfs.zed.settings = {
  ZED_EMAIL_ADDR = [ "root" ];
  ZED_NOTIFY_VERBOSE = true;
};

Above, ZED_EMAIL_ADDR is set to root, which most people will have an alias for in their mailer. You can change it to directly mail you: ZED_EMAIL_ADDR = [ "you@example.com" ];

ZED pulls in mailutils and runs mail by default, but you can override it with ZED_EMAIL_PROG. If using msmtp, you may need ZED_EMAIL_PROG = "${pkgs.msmtp}/bin/msmtp";.

You can customize the mail command with ZED_EMAIL_OPTS. For example, if your upstream mail server requires a certain FROM address: ZED_EMAIL_OPTS = "-r 'noreply@example.com' -s '@SUBJECT@' @ADDRESS@";