diff --git a/weed/topology/volume_layout_avg_fill_test.go b/weed/topology/volume_layout_avg_fill_test.go
new file mode 100644
index 0000000000..a82d983fa0
--- /dev/null
+++ b/weed/topology/volume_layout_avg_fill_test.go
@@ -0,0 +1,138 @@
+package topology
+
+import (
+	"math"
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+func TestWritableAvgFill(t *testing.T) {
+	// 3 writable volumes at 20%, 50%, 80% fill (sizes 2000, 5000, 8000 of
+	// limit 10000): average fill is 50%.
+	layout := `
+{
+  "dc1":{
+    "rack1":{
+      "server1":{
+        "volumes":[
+          {"id":1, "size":2000, "replication":"000"},
+          {"id":2, "size":5000, "replication":"000"},
+          {"id":3, "size":8000, "replication":"000"}
+        ],
+        "limit":10
+      }
+    }
+  }
+}
+`
+	_, vl := setupPickTest(t, layout, 10000)
+
+	avgFill, writableCount := vl.WritableAvgFill()
+	if writableCount != 3 {
+		t.Fatalf("expected 3 writable volumes, got %d", writableCount)
+	}
+	if math.Abs(avgFill-0.5) > 1e-9 {
+		t.Errorf("expected avg fill 0.5, got %v", avgFill)
+	}
+}
+
+func TestWritableAvgFillNoWritables(t *testing.T) {
+	layout := `
+{
+  "dc1":{
+    "rack1":{
+      "server1":{
+        "volumes":[
+          {"id":1, "size":2000, "replication":"000"}
+        ],
+        "limit":10
+      }
+    }
+  }
+}
+`
+	_, vl := setupPickTest(t, layout, 10000)
+
+	vl.accessLock.Lock()
+	vl.removeFromWritable(1)
+	vl.accessLock.Unlock()
+
+	avgFill, writableCount := vl.WritableAvgFill()
+	if writableCount != 0 || avgFill != 0 {
+		t.Errorf("expected (0, 0) with no writables, got (%v, %d)", avgFill, writableCount)
+	}
+}
+
+// A writable volume without a size-tracking entry yet — freshly created, no
+// heartbeat size report so far — counts as empty, not as an error.
+func TestWritableAvgFillUntrackedVolumeCountsAsEmpty(t *testing.T) {
+	layout := `
+{
+  "dc1":{
+    "rack1":{
+      "server1":{
+        "volumes":[
+          {"id":1, "size":5000, "replication":"000"},
+          {"id":2, "size":5000, "replication":"000"}
+        ],
+        "limit":10
+      }
+    }
+  }
+}
+`
+	_, vl := setupPickTest(t, layout, 10000)
+
+	vl.accessLock.Lock()
+	delete(vl.sizeTracking, 2)
+	vl.accessLock.Unlock()
+
+	avgFill, writableCount := vl.WritableAvgFill()
+	if writableCount != 2 {
+		t.Fatalf("expected 2 writable volumes, got %d", writableCount)
+	}
+	if math.Abs(avgFill-0.25) > 1e-9 {
+		t.Errorf("expected avg fill 0.25 (one of two volumes empty), got %v", avgFill)
+	}
+}
+
+func TestFindVolumeLayout(t *testing.T) {
+	layout := `
+{
+  "dc1":{
+    "rack1":{
+      "server1":{
+        "volumes":[
+          {"id":1, "size":2000, "collection":"colA", "replication":"000"}
+        ],
+        "limit":10
+      }
+    }
+  }
+}
+`
+	topo := setupWithLimit(t, layout, 10000)
+	rp, _ := super_block.NewReplicaPlacementFromString("000")
+
+	// Existing layout is found.
+	if vl := topo.FindVolumeLayout("colA", rp, needle.EMPTY_TTL, types.HardDriveType); vl == nil {
+		t.Error("expected layout for colA to be found")
+	}
+
+	// A missing collection resolves to nil and must not be created.
+	if vl := topo.FindVolumeLayout("noSuchCollection", rp, needle.EMPTY_TTL, types.HardDriveType); vl != nil {
+		t.Error("expected nil layout for unknown collection")
+	}
+	if _, ok := topo.collectionMap.Find("noSuchCollection"); ok {
+		t.Error("FindVolumeLayout must not create the collection")
+	}
+
+	// A missing layout within an existing collection also resolves to nil.
+	rpOther, _ := super_block.NewReplicaPlacementFromString("001")
+	if vl := topo.FindVolumeLayout("colA", rpOther, needle.EMPTY_TTL, types.HardDriveType); vl != nil {
+		t.Error("expected nil layout for unused replication")
+	}
+}