{
  config,
  lib,
  pkgs,
  service_configs,
  ...
}:
let
  hddTuneIosched = pkgs.writeShellScript "hdd-tune-iosched" ''
    # Called by udev with the partition kernel name (e.g. sdb1).
    # Derives the parent disk and applies mq-deadline iosched params.
    parent=''${1%%[0-9]*}
    dev="/sys/block/$parent"
    [ -d "$dev/queue/iosched" ] || exit 0
    echo 500 > "$dev/queue/iosched/read_expire"
    echo 15000 > "$dev/queue/iosched/write_expire"
    echo 128 > "$dev/queue/iosched/fifo_batch"
    echo 16 > "$dev/queue/iosched/writes_starved"
    echo 4096 > "$dev/queue/max_sectors_kb" 2>/dev/null || true
  '';
in
{
  boot.initrd.availableKernelModules = [
    "xhci_pci"
    "ahci"
    "usb_storage"
    "usbhid"
    "sd_mod"
  ];
  boot.initrd.kernelModules = [ "dm-snapshot" ];
  boot.kernelModules = [ "kvm-amd" ];
  boot.extraModulePackages = [ ];

  swapDevices = [ ];

  hardware.cpu.amd.updateMicrocode = true;
  hardware.enableRedistributableFirmware = true;

  # HDD I/O tuning for torrent seeding workload (high-concurrency random reads)
  # sharing the pool with latency-sensitive sequential reads (Jellyfin playback).
  #
  # mq-deadline sorts requests into elevator sweeps, reducing seek distance.
  # read_expire=500ms keeps reads bounded so a Jellyfin segment can't queue for
  # seconds behind a torrent burst; write_expire=15s lets the scheduler batch
  # writes for coalescence (torrent writes are async and tolerate delay).
  # The bulk of read coalescence already happens above the scheduler via ZFS
  # aggregation (zfs_vdev_aggregation_limit=4M, read_gap_limit=128K,
  # async_read_max=32), so the scheduler deadline only needs to be large enough
  # to keep the elevator sweep coherent -- 500ms is plenty on rotational disks.
  # fifo_batch=128 keeps sweeps long; writes_starved=16 heavily favors reads.
  # 4 MiB readahead matches libtorrent piece extent affinity for sequential prefetch.
  #
  # The NixOS ZFS module hardcodes a udev rule that forces scheduler="none" on all
  # ZFS member partitions' parent disks (on both add AND change events). We counter
  # it with lib.mkAfter so our rule appears after theirs in 99-local.rules — our
  # rule matches the same partition events and sets mq-deadline back, then a RUN
  # script applies the iosched params. Only targets rotational, non-removable disks
  # (i.e. HDDs, not SSDs or USB).
  services.udev.extraRules = lib.mkAfter ''
    ACTION=="add|change", KERNEL=="sd[a-z]*[0-9]*", ENV{ID_FS_TYPE}=="zfs_member", ATTR{../queue/rotational}=="1", ATTR{../removable}=="0", ATTR{../queue/scheduler}="mq-deadline", ATTR{../queue/read_ahead_kb}="4096", ATTR{../queue/nr_requests}="512", RUN+="${hddTuneIosched} %k"
  '';
}