drbd: Control the actual resync rate based on the queuing delay of data packets

In a setup with a high bandwidth and high latency network, eventually
involving deep queues in routers, it is beneficial to only fill those
queues up to an limited extend with resync data.
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_worker.c

@@ -414,6 +414,18 @@ void resync_timer_fn(unsigned long data)
 		drbd_queue_work(&mdev->data.work, &mdev->resync_work);
 }
 
+static int calc_resync_rate(struct drbd_conf *mdev)
+{
+	int d = mdev->data_delay / 1000; /* us -> ms */
+	int td = mdev->sync_conf.throttle_th * 100;  /* 0.1s -> ms */
+	int hd = mdev->sync_conf.hold_off_th * 100;  /* 0.1s -> ms */
+	int cr = mdev->sync_conf.rate;
+
+	return d <= td ? cr :
+		d >= hd ? 0 :
+		cr + (cr * (td - d) / (hd - td));
+}
+
 int w_make_resync_request(struct drbd_conf *mdev,
 		struct drbd_work *w, int cancel)
 {
@@ -446,7 +458,8 @@ int w_make_resync_request(struct drbd_conf *mdev,
 		return 1;
 	}
 
-	number = SLEEP_TIME * mdev->sync_conf.rate / ((BM_BLOCK_SIZE/1024)*HZ);
+	mdev->c_sync_rate = calc_resync_rate(mdev);
+	number = SLEEP_TIME * mdev->c_sync_rate  / ((BM_BLOCK_SIZE / 1024) * HZ);
 	pe = atomic_read(&mdev->rs_pending_cnt);
 
 	mutex_lock(&mdev->data.mutex);