qbt: enbiggen aggregation of reads

services/qbittorrent.nix

@@ -125,9 +125,22 @@ in
         FilePoolSize = 500; # keep more files open to reduce open/close overhead
         AioThreads = 24; # 6 cores * 4; better disk I/O parallelism
 
+        # Send buffer watermarks control how much upload data qBittorrent pre-reads
+        # from disk per peer. Pushed well above libtorrent defaults (and above
+        # high_performance_seed's 3 MiB) to let the disk I/O thread issue larger,
+        # less-frequent preadv() calls. Larger watermarks give ZFS's vdev aggregator
+        # (4 MiB limit, 128 KiB gap) more contiguous requests to merge per sweep.
+        # Memory cost is roughly watermark × active-peer-count; 6 MiB × a few hundred
+        # peers is well within muffin's RAM budget.
         SendBufferLowWatermark = 512; # 512 KiB -- trigger reads sooner to prevent upload stalls
-        SendBufferWatermark = 3072; # 3 MiB -- matches high_performance_seed
-        SendBufferWatermarkFactor = 150; # percent -- matches high_performance_seed
+        SendBufferWatermark = 6144; # 6 MiB -- bigger pre-reads per peer than high_performance_seed's 3 MiB
+        SendBufferWatermarkFactor = 200; # percent -- scale watermark more aggressively with upload rate
+
+        # Maximum outstanding block requests per peer (libtorrent max_out_request_queue).
+        # Default 500. Tripled so libtorrent's disk I/O thread sees deeper per-peer
+        # request queues, giving it more contiguous blocks to coalesce into single
+        # preadv() calls before issuing them to the kernel.
+        RequestQueueSize = 1500;
       };
 
       Network = {