diff --git a/go/topology/collection.go b/go/topology/collection.go
index 3d7bb7e0e..f8217a7ff 100644
--- a/go/topology/collection.go
+++ b/go/topology/collection.go
@@ -35,7 +35,7 @@ func (c *Collection) GetOrCreateVolumeLayout(ttl *storage.TTL) *VolumeLayout {
 	return vl.(*VolumeLayout)
 }
 
-func (c *Collection) Lookup(vid storage.VolumeId) []*DataNode {
+func (c *Collection) Lookup(vid storage.VolumeId) *VolumeLocationList {
 	for _, vl := range c.storageType2VolumeLayout.Items {
 		if vl != nil {
 			if list := vl.(*VolumeLayout).Lookup(vid); list != nil {
diff --git a/go/topology/topology.go b/go/topology/topology.go
index 6cdd1e1fa..410a1c70e 100644
--- a/go/topology/topology.go
+++ b/go/topology/topology.go
@@ -90,7 +90,7 @@ func (t *Topology) loadConfiguration(configurationFile string) error {
 	return nil
 }
 
-func (t *Topology) Lookup(collection string, vid storage.VolumeId) []*DataNode {
+func (t *Topology) Lookup(collection string, vid storage.VolumeId) *VolumeLocationList {
 	//maybe an issue if lots of collections?
 	if collection == "" {
 		for _, c := range t.collectionMap.Items {
diff --git a/go/topology/topology_event_handling.go b/go/topology/topology_event_handling.go
index 2bb2a9d66..8ad8fc67e 100644
--- a/go/topology/topology_event_handling.go
+++ b/go/topology/topology_event_handling.go
@@ -23,6 +23,7 @@ func (t *Topology) StartRefreshWritableVolumes(garbageThreshold string) {
 		if t.IsLeader() {
 			for range c {
 				t.Vacuum(garbageThreshold)
+				t.CheckReplicate()
 			}
 		}
 	}(garbageThreshold)
diff --git a/go/topology/topology_replicate.go b/go/topology/topology_replicate.go
index 000d6ef4f..5a0186be9 100644
--- a/go/topology/topology_replicate.go
+++ b/go/topology/topology_replicate.go
@@ -1,38 +1,138 @@
 package topology
 
 import (
+	"container/list"
+	"fmt"
+	"time"
+
 	"github.com/chrislusf/seaweedfs/go/glog"
 	"github.com/chrislusf/seaweedfs/go/storage"
 )
 
-func (t *Topology) CheckReplicate() int {
-	glog.V(0).Infoln("Start replicate checker on demand")
+const ReplicateTaskTimeout = time.Hour
+
+type ReplicateTask struct {
+	Vid        storage.VolumeId
+	Collection string
+	SrcDN      *DataNode
+	DstDN      *DataNode
+}
+
+func planReplicateTasks(t *Topology) (tasks []*ReplicateTask) {
 	for _, col := range t.collectionMap.Items {
 		c := col.(*Collection)
 		glog.V(0).Infoln("checking replicate on collection:", c.Name)
+		growOption := &VolumeGrowOption{ReplicaPlacement: c.rp}
 		for _, vl := range c.storageType2VolumeLayout.Items {
 			if vl != nil {
 				volumeLayout := vl.(*VolumeLayout)
-				copyCount := volumeLayout.rp.GetCopyCount()
 				for vid, locationList := range volumeLayout.vid2location {
-					if locationList.Length() < copyCount {
-						//set volume readonly
-						glog.V(0).Infoln("replicate volume :", vid)
-						SetVolumeReadonly(locationList, vid.String(), true)
-
+					rp1 := locationList.CalcReplicaPlacement()
+					if rp1.Compare(volumeLayout.rp) >= 0 {
+						continue
+					}
+					if additionServers, e := FindEmptySlotsForOneVolume(t, growOption, locationList); e == nil {
+						for _, s := range additionServers {
+							s.UpAdjustPlannedVolumeCountDelta(1)
+							rt := &ReplicateTask{
+								Vid:        vid,
+								Collection: c.Name,
+								SrcDN:      locationList.PickForRead(),
+								DstDN:      s,
+							}
+							tasks = append(tasks, rt)
+							glog.V(0).Infof("add replicate task, vid: %v, src: %s, dst: %s", vid, rt.SrcDN.Url(), rt.DstDN.Url())
+						}
+					} else {
+						glog.V(0).Infof("find empty slots error, vid: %v, rp: %s => %s, %v", vid, rp1.String(), volumeLayout.rp.String(), e)
 					}
 				}
 			}
 		}
 	}
-	return 0
+	return
 }
 
-func (t *Topology) doReplicate(vl *VolumeLayout, vid storage.VolumeId) {
-	locationList := vl.vid2location[vid]
-	if !SetVolumeReadonly(locationList, vid.String(), true) {
-		return
+func (topo *Topology) CheckReplicate() {
+	glog.V(1).Infoln("Start replicate checker on demand")
+	busyDataNodes := make(map[*DataNode]int)
+	taskCount := 0
+	taskQueue := list.New()
+	for _, t := range planReplicateTasks(topo) {
+		taskQueue.PushBack(t)
+		taskCount++
 	}
-	defer SetVolumeReadonly(locationList, vid.String(), false)
+	taskChan := make(chan *ReplicateTask)
+	for taskCount > 0 {
+	TaskQueueLoop:
+		for e := taskQueue.Front(); e != nil; e = e.Next() {
+			task := e.Value.(*ReplicateTask)
+			//only one task will run on the data node
+			dns := task.WorkingDataNodes()
+			for _, dn := range dns {
+				if busyDataNodes[dn] > 0 {
+					continue TaskQueueLoop
+				}
+			}
+			for _, dn := range dns {
+				busyDataNodes[dn]++
+			}
+			go func(t *ReplicateTask) {
+				if e := t.Run(topo); e != nil {
+					glog.V(0).Infof("ReplicateTask run error, vid: %v, dst: %s. %v", t.Vid, t.DstDN.Url(), e)
+				} else {
+					glog.V(2).Infof("ReplicateTask finished, vid: %v, dst: %s", t.Vid, t.DstDN.Url())
+
+				}
+				taskChan <- t
+			}(task)
+			taskQueue.Remove(e)
+
+		}
 
+		finishedTask := <-taskChan
+		for _, dn := range finishedTask.WorkingDataNodes() {
+			if busyDataNodes[dn] > 0 {
+				busyDataNodes[dn]--
+			}
+		}
+		taskCount--
+		finishedTask.DstDN.UpAdjustPlannedVolumeCountDelta(-1)
+	}
+	glog.V(1).Infoln("finish replicate check.")
+}
+
+func (t *ReplicateTask) Run(topo *Topology) error {
+	//is lookup thread safe?
+	locationList := topo.Lookup(t.Collection, t.Vid)
+	rp := topo.CollectionSettings.GetReplicaPlacement(t.Collection)
+	if locationList.CalcReplicaPlacement().Compare(rp) >= 0 {
+		glog.V(0).Infof("volume [%v] has right replica placement, rp: %s", t.Vid, rp.String())
+		return nil
+	}
+	if !SetVolumeReadonly(locationList, t.Vid.String(), true) {
+		return fmt.Errorf("set volume readonly failed, vid=%v", t.Vid)
+	}
+	defer SetVolumeReadonly(locationList, t.Vid.String(), false)
+	tc, e := storage.NewTaskCli(t.DstDN.Url(), storage.TaskReplica, storage.TaskParams{
+		"volume":     t.Vid.String(),
+		"source":     t.SrcDN.Url(),
+		"collection": t.Collection,
+	})
+	if e != nil {
+		return e
+	}
+	if e = tc.WaitAndQueryResult(ReplicateTaskTimeout); e != nil {
+		tc.Clean()
+		return e
+	}
+	e = tc.Commit()
+	return e
+}
+
+func (t *ReplicateTask) WorkingDataNodes() []*DataNode {
+	return []*DataNode{
+		t.SrcDN,
+		t.DstDN,
+	}
 }
diff --git a/go/topology/volume_layout.go b/go/topology/volume_layout.go
index 8a922f945..2a38fda99 100644
--- a/go/topology/volume_layout.go
+++ b/go/topology/volume_layout.go
@@ -73,9 +73,9 @@ func (vl *VolumeLayout) isWritable(v *storage.VolumeInfo) bool {
 		!v.ReadOnly
 }
 
-func (vl *VolumeLayout) Lookup(vid storage.VolumeId) []*DataNode {
+func (vl *VolumeLayout) Lookup(vid storage.VolumeId) *VolumeLocationList {
 	if location := vl.vid2location[vid]; location != nil {
-		return location.list
+		return location
 	}
 	return nil
 }
diff --git a/go/topology/volume_location_list.go b/go/topology/volume_location_list.go
index 9c7f60881..929d0c8a3 100644
--- a/go/topology/volume_location_list.go
+++ b/go/topology/volume_location_list.go
@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/chrislusf/seaweedfs/go/storage"
+	"math/rand"
 )
 
 type VolumeLocationList struct {
@@ -23,6 +24,14 @@ func (dnll *VolumeLocationList) Head() *DataNode {
 	return dnll.list[0]
 }
 
+func (dnll *VolumeLocationList) PickForRead() *DataNode {
+	return dnll.list[rand.Intn(len(dnll.list))]
+}
+
+func (dnll *VolumeLocationList) AllDataNode() []*DataNode {
+	return dnll.list
+}
+
 func (dnll *VolumeLocationList) Length() int {
 	return len(dnll.list)
 }
diff --git a/go/weed/weed_server/master_server_handlers.go b/go/weed/weed_server/master_server_handlers.go
index 6a5b06c3c..a61dd765d 100644
--- a/go/weed/weed_server/master_server_handlers.go
+++ b/go/weed/weed_server/master_server_handlers.go
@@ -23,10 +23,10 @@ func (ms *MasterServer) lookupVolumeId(vids []string, collection string) (volume
 		}
 		volumeId, err := storage.NewVolumeId(vid)
 		if err == nil {
-			machines := ms.Topo.Lookup(collection, volumeId)
-			if machines != nil {
+			locationList := ms.Topo.Lookup(collection, volumeId)
+			if locationList != nil {
 				var ret operation.Locations
-				for _, dn := range machines {
+				for _, dn := range locationList.AllDataNode() {
 					ret = append(ret, operation.Location{Url: dn.Url(), PublicUrl: dn.PublicUrl})
 				}
 				volumeLocations[vid] = operation.LookupResult{VolumeId: vid, Locations: ret}
diff --git a/go/weed/weed_server/master_server_handlers_admin.go b/go/weed/weed_server/master_server_handlers_admin.go
index 4b7f809ec..4c883ae2a 100644
--- a/go/weed/weed_server/master_server_handlers_admin.go
+++ b/go/weed/weed_server/master_server_handlers_admin.go
@@ -5,7 +5,6 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
-	"math/rand"
 	"net/http"
 	"strconv"
 	"strings"
@@ -124,9 +123,9 @@ func (ms *MasterServer) redirectHandler(w http.ResponseWriter, r *http.Request)
 		debug("parsing error:", err, r.URL.Path)
 		return
 	}
-	machines := ms.Topo.Lookup("", volumeId)
-	if machines != nil && len(machines) > 0 {
-		http.Redirect(w, r, util.NormalizeUrl(machines[rand.Intn(len(machines))].PublicUrl)+r.URL.Path, http.StatusMovedPermanently)
+	locations := ms.Topo.Lookup("", volumeId)
+	if locations != nil && locations.Length() > 0 {
+		http.Redirect(w, r, util.NormalizeUrl(locations.PickForRead().PublicUrl)+r.URL.Path, http.StatusMovedPermanently)
 	} else {
 		writeJsonError(w, r, http.StatusNotFound, fmt.Errorf("volume id %d not found", volumeId))
 	}