fix: handle zero maxResults as no-cap, emit trace after trim, seed empty servers

- When MaxResults is 0 (omitted), treat as no explicit cap instead of
  defaulting to 1; only apply the +1 over-fetch probe when caller
  supplies a positive limit
- Move decision trace emission after hasMore/trim so the trace
  accurately reflects the returned proposals
- Seed serverVolumeCounts from ActiveTopology so servers that have a
  matching disk type but zero volumes are included in the imbalance
  calculation and MinServerCount check

weed/plugin/worker/volume_balance_handler.go

@@ -225,16 +225,14 @@ func (h *VolumeBalanceHandler) Detect(
 
 	clusterInfo := &workertypes.ClusterInfo{ActiveTopology: activeTopology}
 	maxResults := int(request.MaxResults)
-	if maxResults <= 0 {
-		maxResults = 1
+	detectionLimit := maxResults
+	if maxResults > 0 {
+		detectionLimit = maxResults + 1 // over-fetch by 1 to detect truncation
 	}
-	results, err := balancetask.Detection(metrics, clusterInfo, workerConfig.TaskConfig, maxResults+1)
+	results, err := balancetask.Detection(metrics, clusterInfo, workerConfig.TaskConfig, detectionLimit)
 	if err != nil {
 		return err
 	}
-	if traceErr := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, workerConfig.TaskConfig, results); traceErr != nil {
-		glog.Warningf("Plugin worker failed to emit volume_balance detection trace: %v", traceErr)
-	}
 
 	hasMore := false
 	if maxResults > 0 && len(results) > maxResults {
@@ -242,6 +240,10 @@ func (h *VolumeBalanceHandler) Detect(
 		results = results[:maxResults]
 	}
 
+	if traceErr := emitVolumeBalanceDetectionDecisionTrace(sender, metrics, workerConfig.TaskConfig, results); traceErr != nil {
+		glog.Warningf("Plugin worker failed to emit volume_balance detection trace: %v", traceErr)
+	}
+
 	proposals := make([]*plugin_pb.JobProposal, 0, len(results))
 	for _, result := range results {
 		proposal, proposalErr := buildVolumeBalanceProposal(result)

weed/worker/tasks/balance/detection.go

@@ -15,6 +15,7 @@ import (
 
 // Detection implements the detection logic for balance tasks.
 // maxResults limits how many balance operations are returned per invocation.
+// A non-positive maxResults means no explicit limit (uses a large default).
 func Detection(metrics []*types.VolumeHealthMetrics, clusterInfo *types.ClusterInfo, config base.TaskConfig, maxResults int) ([]*types.TaskDetectionResult, error) {
 	if !config.IsEnabled() {
 		return nil, nil
@@ -23,7 +24,7 @@ func Detection(metrics []*types.VolumeHealthMetrics, clusterInfo *types.ClusterI
 	balanceConfig := config.(*Config)
 
 	if maxResults <= 0 {
-		maxResults = 1
+		maxResults = math.MaxInt32
 	}
 
 	// Group volumes by disk type to ensure we compare apples to apples
@@ -57,8 +58,26 @@ func detectForDiskType(diskType string, diskMetrics []*types.VolumeHealthMetrics
 		return nil
 	}
 
-	// Analyze volume distribution across servers
+	// Analyze volume distribution across servers.
+	// Seed from ActiveTopology so servers with matching disk type but zero
+	// volumes are included in the count and imbalance calculation.
 	serverVolumeCounts := make(map[string]int)
+	if clusterInfo != nil && clusterInfo.ActiveTopology != nil {
+		topologyInfo := clusterInfo.ActiveTopology.GetTopologyInfo()
+		if topologyInfo != nil {
+			for _, dc := range topologyInfo.DataCenterInfos {
+				for _, rack := range dc.RackInfos {
+					for _, node := range rack.DataNodeInfos {
+						for diskTypeName := range node.DiskInfos {
+							if diskTypeName == diskType {
+								serverVolumeCounts[node.Id] = 0
+							}
+						}
+					}
+				}
+			}
+		}
+	}
 	for _, metric := range diskMetrics {
 		serverVolumeCounts[metric.Server]++
 	}