diff --git a/weed/topology/crowded_writable_test.go b/weed/topology/crowded_writable_test.go
new file mode 100644
index 000000000..ba87bcb6d
--- /dev/null
+++ b/weed/topology/crowded_writable_test.go
@@ -0,0 +1,75 @@
+package topology
+
+import (
+	"testing"
+	"time"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+// Crowding asks for more room to write into, so a read-only volume has nothing
+// to say about it. Growth already discounts them by intersecting with the
+// writable list, so an entry for one is weight and nothing else.
+func TestCrowdedVolumesAreOnesThatCanTakeWrites(t *testing.T) {
+	topo := NewTopology("crowd", nil, 10000, 5, false)
+	dn := topo.GetOrCreateDataCenter("dc1").GetOrCreateRack("rack1").
+		GetOrCreateDataNode("127.0.0.1", 8080, 18080, "", "", map[string]uint32{"": 100})
+
+	// Both are past the growth threshold; only one can be written to.
+	topo.SyncDataNodeRegistration([]*master_pb.VolumeInformationMessage{
+		{Id: 1, Size: 9500, Collection: "c", Version: 3},
+		{Id: 2, Size: 9500, Collection: "c", Version: 3, ReadOnly: true},
+	}, dn)
+	dn.LastSeen = time.Now().Unix()
+
+	// The channels are unbuffered and normally drained by the writable-refresh
+	// loop, which is not running here.
+	crowded := map[needle.VolumeId]bool{}
+	stop, done := make(chan struct{}), make(chan struct{})
+	go func() {
+		defer close(done)
+		for {
+			select {
+			case v := <-topo.chanCrowdedVolumes:
+				crowded[v.Id] = true
+			case <-topo.chanFullVolumes:
+			case <-stop:
+				return
+			}
+		}
+	}()
+	// The channels are unbuffered, so every send has been received by the time
+	// this returns; waiting for the collector to finish covers the recording.
+	topo.CollectDeadNodeAndFullVolumes(time.Now().Unix()-1, topo.volumeSizeLimit, 0.9)
+	close(stop)
+	<-done // the collector owns the map until it returns
+
+	if !crowded[needle.VolumeId(1)] {
+		t.Error("a writable volume past the threshold was not reported crowded")
+	}
+	if crowded[needle.VolumeId(2)] {
+		t.Error("a read-only volume was reported crowded, which only adds an entry growth then discounts")
+	}
+}
+
+// The count growth decides on has to be unchanged by this.
+func TestCrowdedCountStillMatchesWritables(t *testing.T) {
+	topo := NewTopology("crowd", nil, 10000, 5, false)
+	dn := topo.GetOrCreateDataCenter("dc1").GetOrCreateRack("rack1").
+		GetOrCreateDataNode("127.0.0.1", 8080, 18080, "", "", map[string]uint32{"": 100})
+	topo.SyncDataNodeRegistration([]*master_pb.VolumeInformationMessage{
+		{Id: 1, Size: 9500, Collection: "c", Version: 3},
+	}, dn)
+
+	rp, _ := super_block.NewReplicaPlacementFromString("000")
+	vl := topo.GetVolumeLayout("c", rp, needle.EMPTY_TTL, types.HardDriveType)
+	vl.SetVolumeCrowded(needle.VolumeId(1))
+
+	writable, crowded := vl.GetWritableVolumeCount()
+	if writable != 1 || crowded != 1 {
+		t.Errorf("writable=%d crowded=%d, want 1 and 1", writable, crowded)
+	}
+}
diff --git a/weed/topology/node.go b/weed/topology/node.go
index d1d121471..ebbf43946 100644
--- a/weed/topology/node.go
+++ b/weed/topology/node.go
@@ -514,7 +514,11 @@ func (n *NodeImpl) CollectDeadNodeAndFullVolumes(freshThreshHoldUnixTime int64,
 						//fmt.Println("volume",v.Id,"size",v.Size,">",volumeSizeLimit)
 						topo.chanFullVolumes <- v
 					}
-				} else if float64(v.Size) > float64(volumeSizeLimit)*growThreshold {
+				} else if !v.ReadOnly && float64(v.Size) > float64(volumeSizeLimit)*growThreshold {
+					// Crowding asks for more room to write into, which a
+					// read-only volume can never provide. Growth already
+					// discounts them by intersecting with the writable list, so
+					// marking one only costs the entry.
 					topo.chanCrowdedVolumes <- v
 				}
 				copyCount := v.ReplicaPlacement.GetCopyCount()