zfs: tweak settings

modules/zfs.nix

@@ -28,7 +28,7 @@ in
     # HDD pool (hdds) writes larger, sequential I/Os instead of many small syncs.
     # This is a global setting (no per-pool control); the SSD pool (tank) syncs
     # infrequently but handles it fine since SSDs don't suffer from seek overhead.
-    "zfs.zfs_txg_timeout=120"
+    "zfs.zfs_txg_timeout=60"
 
     # Cap ARC to prevent it from claiming memory reserved for hugepages.
     # Without this, ZFS auto-sizes c_max to ~62 GiB on a 64 GiB system,
@@ -39,7 +39,7 @@ in
     # 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_max_active=16"
     "zfs.zfs_vdev_async_read_min_active=4"
 
     # Merge reads within 128 KiB of each other (default 32 KiB). On HDDs, reading a