zfs: tune hdds pool

modules/hardware.nix

@@ -21,4 +21,15 @@
 
   hardware.cpu.amd.updateMicrocode = true;
   hardware.enableRedistributableFirmware = true;
+
+  # HDD I/O tuning for torrent seeding workload (high-concurrency random reads).
+  # mq-deadline sorts requests into elevator sweeps, reducing seek distance.
+  # Aggressive deadlines (15s) let the scheduler accumulate more ops before dispatching,
+  # maximizing coalescence — latency is irrelevant since torrent peers tolerate 30-60s.
+  # fifo_batch=128 keeps sweeps long; writes_starved=16 heavily favors reads.
+  # 4 MiB readahead matches libtorrent piece extent affinity for sequential prefetch.
+  services.udev.extraRules = ''
+    ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="mq-deadline", ATTR{queue/read_ahead_kb}="4096", ATTR{queue/nr_requests}="512"
+    ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ENV{ID_ATA_ROTATION_RATE_RPM}!="0", RUN+="${pkgs.bash}/bin/bash -c 'echo 15000 > /sys$devpath/queue/iosched/read_expire; echo 15000 > /sys$devpath/queue/iosched/write_expire; echo 128 > /sys$devpath/queue/iosched/fifo_batch; echo 16 > /sys$devpath/queue/iosched/writes_starved; echo 4096 > /sys$devpath/queue/max_sectors_kb 2>/dev/null || true'"
+  '';
 }