qbt: DiskIOType = Posix

services/qbittorrent.nix

@@ -111,6 +111,12 @@
         # where ZFS txg commits cause periodic I/O stalls.
         DiskQueueSize = 67108864; # 64MB
 
+        # POSIX-compliant disk I/O: uses pread/pwrite instead of mmap.
+        # On ZFS, mmap forces data into BOTH ARC and Linux page cache (double-caching),
+        # wasting RAM. pread/pwrite goes only through ARC, maximizing its effectiveness.
+        # Saved 26 gb of memory!!
+        DiskIOType = "Posix";
+
         # === Network buffer tuning (from libtorrent high_performance_seed preset) ===
         # "always stuff at least 1 MiB down each peer pipe, to quickly ramp up send rates"
         SendBufferLowWatermark = 1024; # 1MB (KiB) -- matches high_performance_seed