zfs: tune hdds pool

modules/zfs.nix

@@ -10,6 +10,21 @@
 
   boot.kernelParams = [
     "zfs.zfs_txg_timeout=120" # longer TXG open time = larger sequential writes
+
+    # vdev I/O scheduler: feed more concurrent reads to the block scheduler so
+    # mq-deadline has a larger pool of requests to sort and merge into elevator sweeps.
+    # Default async_read_max is 3 — far too few for effective coalescence.
+    # 32 was empirically optimal (64 overwhelmed the drives, 3 gave near-zero merges).
+    "zfs.zfs_vdev_async_read_max_active=32"
+    "zfs.zfs_vdev_async_read_min_active=4"
+
+    # Merge reads within 128 KiB of each other (default 32 KiB). On HDDs, reading a
+    # 128 KiB gap is far cheaper than a mechanical seek (~8 ms).
+    "zfs.zfs_vdev_read_gap_limit=131072"
+
+    # Allow ZFS to aggregate I/Os up to 4 MiB (default 1 MiB), matching the
+    # libtorrent piece extent size for larger sequential disk operations.
+    "zfs.zfs_vdev_aggregation_limit=4194304"
   ];
 
   boot.supportedFilesystems = [ "zfs" ];