ZFS

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

Besides the zfs package (ZFS Filesystem Linux Kernel module) itself there are many packages in the ZFS ecosystem available.

ZFS integrates into NixOS via the boot.zfs and service.zfs options.

Limitations

Latest kernel compatible with ZFS

Newer kernels might not be supported by ZFS yet. If you are running a kernel which is not officially supported by zfs, the module will refuse to evaluate and show an error.

You can pin to a newer kernel version explicitly, but note that this version may be dropped by upstream and in nixpkgs prior to zfs supporting the next version. See Linux kernel for more information.

{
  boot.kernelPackages = pkgs.linuxPackages_latest;
  # OR
  boot.kernelPackages = pkgs.linuxPackages_6_6
}

This snippet will configure the latest compatible kernel. Note that this can over time jump back to old kernel versions because non-lts kernel version get removed over time and their newer replacements might be not supported by zfs yet.

{
  lib,
  pkgs,
  config,
  ...
}:

let
  isUnstable = config.boot.zfs.package == pkgs.zfsUnstable;
  zfsCompatibleKernelPackages = lib.filterAttrs (
    name: kernelPackages:
    (builtins.match "linux_[0-9]+_[0-9]+" name) != null
    && (builtins.tryEval kernelPackages).success
    && (
      (!isUnstable && !kernelPackages.zfs.meta.broken)
      || (isUnstable && !kernelPackages.zfs_unstable.meta.broken)
    )
  ) pkgs.linuxKernel.packages;
  latestKernelPackage = lib.last (
    lib.sort (a: b: (lib.versionOlder a.kernel.version b.kernel.version)) (
      builtins.attrValues zfsCompatibleKernelPackages
    )
  );
in
{
  # Note this might jump back and worth as kernel get added or removed.
  boot.kernelPackages = latestKernelPackage;
}

Partial support for SWAP on ZFS

ZFS does not support swapfiles. SWAP devices can be used instead. Additionally, hibernation is disabled by default due to a high risk of data corruption. Note that even if that pull request is merged, it does not fully mitigate the risk. If you wish to enable hibernation regardless and made sure that not swapfiles on ZFS are used, set boot.zfs.allowHibernation = true.

Zpool not found

If NixOS fails to import the zpool on reboot, you may need to add boot.zfs.devNodes = "/dev/disk/by-path"; or boot.zfs.devNodes = "/dev/disk/by-partuuid"; to your configuration.nix file.

The differences can be tested by running zpool import -d /dev/disk/by-id when none of the pools are discovered, eg. a live iso.

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

Warning: This guide is not endorsed by NixOS and some features like immutable root do not have upstream support and could break on updates. If an issue arises while following this guide, please consult the guides support channels.

One 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:

• Give understandable, easy to follow and close to the standard installation guide instructions
• integrating ZFS into your existing config

Simple NixOS ZFS in root installation

Start from here in the NixOS manual: [1]. Under manual partitioning [2] do this instead:

Partition your disk with your favorite partition tool.

We need the following partitions:

• 1G for boot partition with "boot" as the partition label (also called name in some tools) and ef00 as partition code
• 4G for a swap partition with "swap" as the partition label and 8200 as partition code. We will encrypt this with a random secret on each boot.
• The rest of disk space for zfs with "root" as the partition label and 8300 as partition code (default code)

Reason for swap partition: ZFS does use a caching mechanism that is different from the normal Linux cache infrastructure. In low-memory situations, ZFS therefore might need a bit longer to free up memory from its cache. The swap partition will help with that.

Example with gdisk:

sudo gdisk /dev/nvme0n1
GPT fdisk (gdisk) version 1.0.10
...
# boot partition
Command (? for help): n
Partition number (1-128, default 1): 
First sector (2048-1000215182, default = 2048) or {+-}size{KMGTP}: 
Last sector (2048-1000215182, default = 1000215175) or {+-}size{KMGTP}: +1G
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): ef00
Changed type of partition to 'EFI system partition'

# Swap partition
Command (? for help): n
Partition number (2-128, default 2): 
First sector (2099200-1000215182, default = 2099200) or {+-}size{KMGTP}: 
Last sector (2099200-1000215182, default = 1000215175) or {+-}size{KMGTP}: +4G
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): 8200
Changed type of partition to 'Linux swap'

# root partition
Command (? for help): n
Partition number (3-128, default 3): 
First sector (10487808-1000215182, default = 10487808) or {+-}size{KMGTP}: 
Last sector (10487808-1000215182, default = 1000215175) or {+-}size{KMGTP}: 
Current type is 8300 (Linux filesystem)
Hex code or GUID (L to show codes, Enter = 8300): 
Changed type of partition to 'Linux filesystem'

# write changes
Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y
OK; writing new GUID partition table (GPT) to /dev/nvme0n1.
The operation has completed successfully.

Final partition table

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         2099199   1024.0 MiB  EF00  EFI system partition
   2         2099200        10487807   4.0 GiB     8200  Linux swap
   3        10487808      1000215175   471.9 GiB   8300  Linux filesystem

Let's use variables from now on for simplicity. Get the device ID in /dev/disk/by-id/, in our case here it is nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O

BOOT=/dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part1
SWAP=/dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part2
DISK=/dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part3

'''Make zfs pool with encryption and mount points:'''

'''Note:''' zpool config can significantly affect performance (especially the ashift option) so you may want to do some research. The [https://jrs-s.net/2018/08/17/zfs-tuning-cheat-sheet/ ZFS tuning cheatsheet] or [https://wiki.archlinux.org/title/ZFS#Storage_pools ArchWiki] is a good place to start.

<syntaxhighlight lang="bash">
zpool create -O encryption=on -O keyformat=passphrase -O keylocation=prompt -O compression=zstd -O mountpoint=none -O xattr=sa -O acltype=posixacl -o ashift=12 zpool $DISK
# enter the password to decrypt the pool at boot
Enter new passphrase:
Re-enter new passphrase:

# Create datasets
zfs create zpool/root
zfs create zpool/nix
zfs create zpool/var
zfs create zpool/home

mkdir -p /mnt
mount -t zfs zpool/root /mnt -o zfsutil
mkdir /mnt/nix /mnt/var /mnt/home

mount -t zfs zpool/nix /mnt/nix -o zfsutil
mount -t zfs zpool/var /mnt/var -o zfsutil
mount -t zfs zpool/home /mnt/home -o zfsutil

Output from zpool status:

zpool status
  pool: zpool
 state: ONLINE
...
config:

	NAME                               STATE     READ WRITE CKSUM
	zpool                              ONLINE       0     0     0
	  nvme-eui.0025384b21406566-part2  ONLINE       0     0     0

Format boot partition with fat as filesystem

mkfs.fat -F 32 -n boot $BOOT

Enable swap

mkswap -L swap $SWAP
swapon $SWAP

Installation:

1. Mount boot

mkdir -p /mnt/boot
mount $BOOT /mnt/boot

# Generate the nixos config
nixos-generate-config --root /mnt
...
writing /mnt/etc/nixos/hardware-configuration.nix...
writing /mnt/etc/nixos/configuration.nix...
For more hardware-specific settings, see https://github.com/NixOS/nixos-hardware.

Now edit the configuration.nix that was just created in /mnt/etc/nixos/configuration.nix and make sure to have at least the following content in it.

{
...
  # Boot loader config for configuration.nix:
  boot.loader.systemd-boot.enable = true;

  # for local disks that are not shared over the network, we don't need this to be random
  networking.hostId = "8425e349";
...

Now check the hardware-configuration.nix in /mnt/etc/nixos/hardware-configuration.nix and add whats missing e.g. options = [ "zfsutil" ] for all filesystems except boot and randomEncryption = true; for the swap partition. Also change the generated swap device to the partition we created e.g. /dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part2 in this case and /dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part1 for boot.

...
  fileSystems."/" = { 
    device = "zpool/root";
    fsType = "zfs";
    # the zfsutil option is needed when mounting zfs datasets without "legacy" mountpoints
    options = [ "zfsutil" ];
  };

  fileSystems."/nix" = { 
    device = "zpool/nix";
    fsType = "zfs";
    options = [ "zfsutil" ];
  };

  fileSystems."/var" = { 
    device = "zpool/var";
    fsType = "zfs";
    options = [ "zfsutil" ];
  };

  fileSystems."/home" = {
    device = "zpool/home";
    fsType = "zfs";
    options = [ "zfsutil" ];
  };

  fileSystems."/boot" = { 
   device = "/dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part1";
   fsType = "vfat";
  };

  swapDevices = [{
    device = "/dev/disk/by-id/nvme-SKHynix_HFS512GDE9X081N_FNB6N634510106K5O-part2";
    randomEncryption = true;
  }];
}

Now you may install nixos with nixos-install

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";
};

Zpool created with bus-based disk names

If you used bus-based disk names in the zpool create command, e.g., /dev/sda, NixOS may run into issues importing the pool if the names change. Even if the pool is able to be mounted (with boot.zfs.devNodes = "/dev/disk/by-partuuid"; set), this may manifest as a FAULTED disk and a DEGRADED pool reported by zpool status. The fix is to re-import the pool using disk IDs:

# zpool export zpool_name
# zpool import -d /dev/disk/by-id zpool_name

The import setting is reflected in /etc/zfs/zpool.cache, so it should persist through subsequent boots.

Zpool created with disk IDs

If you used disk IDs to refer to disks in the zpool create command, e.g., /dev/disk/by-id, then NixOS may consistently fail to import the pool unless boot.zfs.devNodes = "/dev/disk/by-id" is also set.

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).

Remote unlock

Unlock encrypted zfs via ssh on boot

Note: As of 22.05, rebuilding your config with the below directions may result in a situation where, if you want to revert the changes, you may need to do some pretty hairy nix-store manipulation to be able to successfully rebuild, see https://github.com/NixOS/nixpkgs/issues/101462#issuecomment-1172926129

In case you want unlock a machine remotely (after an update), having an ssh service in initrd for the password prompt is handy:

boot = {
  initrd.network = {
    # This will use udhcp to get an ip address.
    # Make sure you have added the kernel module for your network driver to `boot.initrd.availableKernelModules`, 
    # so your initrd can load it!
    # Static ip addresses might be configured using the ip argument in kernel command line:
    # https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt
    enable = true;
    ssh = {
      enable = true;
      # To prevent ssh clients from freaking out because a different host key is used,
      # a different port for ssh is useful (assuming the same host has also a regular sshd running)
      port = 2222; 
      # hostKeys paths must be unquoted strings, otherwise you'll run into issues with boot.initrd.secrets
      # the keys are copied to initrd from the path specified; multiple keys can be set
      # you can generate any number of host keys using 
      # `ssh-keygen -t ed25519 -N "" -f /path/to/ssh_host_ed25519_key`
      hostKeys = [ /path/to/ssh_host_rsa_key ];
      # public ssh key used for login
      authorizedKeys = [ "ssh-rsa AAAA..." ];
    };
  };
};

• In order to use DHCP in the initrd, network manager must not be enabled and networking.useDHCP = true; must be set.
• If your network card isn't started, you'll need to add the according kernel module to the kernel and initrd as well, e.g.

  boot.kernelModules = [ "r8169" ];
  boot.initrd.kernelModules = [ "r8169" ];
  

After that you can unlock your datasets using the following ssh command:

ssh -p 2222 root@host "zpool import -a; zfs load-key -a && killall zfs"

Alternatively you could also add the commands as postCommands to your configuration.nix, then you just have to ssh into the initrd:

boot = {
  initrd.network = {
    postCommands = ''
    # Import all pools
    zpool import -a
    # Or import selected pools
    zpool import pool2
    zpool import pool3
    zpool import pool4
    # Add the load-key command to the .profile
    echo "zfs load-key -a; killall zfs" >> /root/.profile
    '';
  };
};

After that you can unlock your datasets using the following ssh command:

ssh -p 2222 root@host

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@";