From 27c74a7e66558a4f9ce0d10621606dfed98a3abb Mon Sep 17 00:00:00 2001
From: Chris Lu <chris.lu@gmail.com>
Date: Sun, 2 Mar 2014 22:16:54 -0800
Subject: [PATCH] Major: change replication_type to ReplicaPlacement, hopefully
 cleaner code works for 9 possible ReplicaPlacement xyz x : number of copies
 on other data centers y : number of copies on other racks z : number of
 copies on current rack x y z each can be 0,1,2

Minor:
weed server "-mdir" default to "-dir" if empty
---
 go/operation/list_masters.go                  |   6 +-
 go/replication/allocate_volume.go             |   4 +-
 go/replication/volume_growth.go               | 246 ++++++------------
 go/replication/volume_growth_test.go          |  28 +-
 go/storage/cdb_map.go                         |   4 +-
 go/storage/compact_map_perf_test.go           |   2 +-
 go/storage/replica_placement.go               |  61 +++++
 go/storage/replication_type.go                | 123 ---------
 go/storage/store.go                           |  18 +-
 go/storage/volume.go                          |  18 +-
 go/storage/volume_info.go                     |   2 +-
 go/topology/collection.go                     |  14 +-
 go/topology/data_center.go                    |   1 +
 go/topology/node.go                           |  69 ++++-
 go/topology/node_list.go                      |  83 ------
 go/topology/node_list_test.go                 |  60 -----
 go/topology/rack.go                           |   1 +
 go/topology/topo_test.go                      |  14 -
 go/topology/topology.go                       |  20 +-
 go/topology/topology_event_handling.go        |   6 +-
 go/topology/volume_layout.go                  |  16 +-
 go/util/file_util.go                          |   2 +-
 go/weed/compact.go                            |   2 +-
 go/weed/download.go                           |   2 +-
 go/weed/export.go                             |   4 +-
 go/weed/master.go                             |  26 +-
 go/weed/server.go                             |  39 +--
 go/weed/version.go                            |   2 +-
 go/weed/weed_server/master_server.go          |  30 +--
 go/weed/weed_server/master_server_handlers.go |  24 +-
 go/weed/weed_server/volume_server.go          |   2 +-
 note/replication.txt                          |  37 ++-
 32 files changed, 371 insertions(+), 595 deletions(-)
 create mode 100644 go/storage/replica_placement.go
 delete mode 100644 go/storage/replication_type.go
 delete mode 100644 go/topology/node_list.go
 delete mode 100644 go/topology/node_list_test.go

diff --git a/go/operation/list_masters.go b/go/operation/list_masters.go
index 05235aed0..ade975c71 100644
--- a/go/operation/list_masters.go
+++ b/go/operation/list_masters.go
@@ -23,5 +23,9 @@ func ListMasters(server string) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	return ret.Peers, nil
+	masters := ret.Peers
+	if ret.IsLeader {
+		masters = append(masters, ret.Leader)
+	}
+	return masters, nil
 }
diff --git a/go/replication/allocate_volume.go b/go/replication/allocate_volume.go
index 0f5ebc00f..fb40c6353 100644
--- a/go/replication/allocate_volume.go
+++ b/go/replication/allocate_volume.go
@@ -13,11 +13,11 @@ type AllocateVolumeResult struct {
 	Error string
 }
 
-func AllocateVolume(dn *topology.DataNode, vid storage.VolumeId, collection string, repType storage.ReplicationType) error {
+func AllocateVolume(dn *topology.DataNode, vid storage.VolumeId, collection string, rp *storage.ReplicaPlacement) error {
 	values := make(url.Values)
 	values.Add("volume", vid.String())
 	values.Add("collection", collection)
-	values.Add("replication", repType.String())
+	values.Add("replication", rp.String())
 	jsonBlob, err := util.Post("http://"+dn.PublicUrl+"/admin/assign_volume", values)
 	if err != nil {
 		return err
diff --git a/go/replication/volume_growth.go b/go/replication/volume_growth.go
index d7d1c90bd..8466b149f 100644
--- a/go/replication/volume_growth.go
+++ b/go/replication/volume_growth.go
@@ -5,7 +5,6 @@ import (
 	"code.google.com/p/weed-fs/go/storage"
 	"code.google.com/p/weed-fs/go/topology"
 	"errors"
-	"fmt"
 	"math/rand"
 	"sync"
 )
@@ -19,188 +18,115 @@ This package is created to resolve these replica placement issues:
 */
 
 type VolumeGrowth struct {
-	copy1factor int
-	copy2factor int
-	copy3factor int
-	copyAll     int
-
 	accessLock sync.Mutex
 }
 
 func NewDefaultVolumeGrowth() *VolumeGrowth {
-	return &VolumeGrowth{copy1factor: 7, copy2factor: 6, copy3factor: 3}
+	return &VolumeGrowth{}
 }
 
-func (vg *VolumeGrowth) AutomaticGrowByType(collection string, repType storage.ReplicationType, dataCenter string, topo *topology.Topology) (count int, err error) {
-	factor := 1
-	switch repType {
-	case storage.Copy000:
-		factor = 1
-		count, err = vg.GrowByCountAndType(vg.copy1factor, collection, repType, dataCenter, topo)
-	case storage.Copy001:
-		factor = 2
-		count, err = vg.GrowByCountAndType(vg.copy2factor, collection, repType, dataCenter, topo)
-	case storage.Copy010:
-		factor = 2
-		count, err = vg.GrowByCountAndType(vg.copy2factor, collection, repType, dataCenter, topo)
-	case storage.Copy100:
-		factor = 2
-		count, err = vg.GrowByCountAndType(vg.copy2factor, collection, repType, dataCenter, topo)
-	case storage.Copy110:
-		factor = 3
-		count, err = vg.GrowByCountAndType(vg.copy3factor, collection, repType, dataCenter, topo)
-	case storage.Copy200:
-		factor = 3
-		count, err = vg.GrowByCountAndType(vg.copy3factor, collection, repType, dataCenter, topo)
+// one replication type may need rp.GetCopyCount() actual volumes
+// given copyCount, how many logical volumes to create
+func (vg *VolumeGrowth) findVolumeCount(copyCount int) (count int) {
+	switch copyCount {
+	case 1:
+		count = 7
+	case 2:
+		count = 6
+	case 3:
+		count = 3
 	default:
-		err = errors.New("Unknown Replication Type!")
+		count = 1
 	}
-	if count > 0 && count%factor == 0 {
+	return
+}
+
+func (vg *VolumeGrowth) AutomaticGrowByType(collection string, rp *storage.ReplicaPlacement, preferredDataCenter string, topo *topology.Topology) (count int, err error) {
+	count, err = vg.GrowByCountAndType(vg.findVolumeCount(rp.GetCopyCount()), collection, rp, preferredDataCenter, topo)
+	if count > 0 && count%rp.GetCopyCount() == 0 {
 		return count, nil
 	}
 	return count, err
 }
-func (vg *VolumeGrowth) GrowByCountAndType(count int, collection string, repType storage.ReplicationType, dataCenter string, topo *topology.Topology) (counter int, err error) {
+func (vg *VolumeGrowth) GrowByCountAndType(targetCount int, collection string, rp *storage.ReplicaPlacement, preferredDataCenter string, topo *topology.Topology) (counter int, err error) {
 	vg.accessLock.Lock()
 	defer vg.accessLock.Unlock()
 
-	counter = 0
-	switch repType {
-	case storage.Copy000:
-		for i := 0; i < count; i++ {
-			if ok, server, vid := topo.RandomlyReserveOneVolume(dataCenter); ok {
-				if err = vg.grow(topo, *vid, collection, repType, server); err == nil {
-					counter++
-				} else {
-					return counter, err
-				}
-			} else {
-				return counter, fmt.Errorf("Failed to grown volume for data center %s", dataCenter)
-			}
-		}
-	case storage.Copy001:
-		for i := 0; i < count; i++ {
-			//randomly pick one server from the datacenter, and then choose from the same rack
-			if ok, server1, vid := topo.RandomlyReserveOneVolume(dataCenter); ok {
-				rack := server1.Parent()
-				exclusion := make(map[string]topology.Node)
-				exclusion[server1.String()] = server1
-				newNodeList := topology.NewNodeList(rack.Children(), exclusion)
-				if newNodeList.FreeSpace() > 0 {
-					if ok2, server2 := newNodeList.ReserveOneVolume(rand.Intn(newNodeList.FreeSpace()), *vid); ok2 {
-						if err = vg.grow(topo, *vid, collection, repType, server1, server2); err == nil {
-							counter++
-						}
-					}
-				}
-			}
-		}
-	case storage.Copy010:
-		for i := 0; i < count; i++ {
-			//randomly pick one server from the datacenter, and then choose from the a different rack
-			if ok, server1, vid := topo.RandomlyReserveOneVolume(dataCenter); ok {
-				rack := server1.Parent()
-				dc := rack.Parent()
-				exclusion := make(map[string]topology.Node)
-				exclusion[rack.String()] = rack
-				newNodeList := topology.NewNodeList(dc.Children(), exclusion)
-				if newNodeList.FreeSpace() > 0 {
-					if ok2, server2 := newNodeList.ReserveOneVolume(rand.Intn(newNodeList.FreeSpace()), *vid); ok2 {
-						if err = vg.grow(topo, *vid, collection, repType, server1, server2); err == nil {
-							counter++
-						}
-					}
-				}
-			}
+	for i := 0; i < targetCount; i++ {
+		if c, e := vg.findAndGrow(topo, preferredDataCenter, collection, rp); e == nil {
+			counter += c
+		} else {
+			return counter, e
 		}
-	case storage.Copy100:
-		for i := 0; i < count; i++ {
-			nl := topology.NewNodeList(topo.Children(), nil)
-			picked, ret := nl.RandomlyPickN(2, 1, dataCenter)
-			vid := topo.NextVolumeId()
-			if ret {
-				var servers []*topology.DataNode
-				for _, n := range picked {
-					if n.FreeSpace() > 0 {
-						if ok, server := n.ReserveOneVolume(rand.Intn(n.FreeSpace()), vid, ""); ok {
-							servers = append(servers, server)
-						}
-					}
-				}
-				if len(servers) == 2 {
-					if err = vg.grow(topo, vid, collection, repType, servers...); err == nil {
-						counter++
-					}
-				}
-			} else {
-				return counter, fmt.Errorf("Failed to grown volume on data center %s and another data center", dataCenter)
-			}
+	}
+	return
+}
+
+func (vg *VolumeGrowth) findAndGrow(topo *topology.Topology, preferredDataCenter string, collection string, rp *storage.ReplicaPlacement) (int, error) {
+	servers, e := vg.findEmptySlotsForOneVolume(topo, preferredDataCenter, rp)
+	if e != nil {
+		return 0, e
+	}
+	vid := topo.NextVolumeId()
+	err := vg.grow(topo, vid, collection, rp, servers...)
+	return len(servers), err
+}
+
+func (vg *VolumeGrowth) findEmptySlotsForOneVolume(topo *topology.Topology, preferredDataCenter string, rp *storage.ReplicaPlacement) (servers []*topology.DataNode, err error) {
+	//find main datacenter and other data centers
+	mainDataCenter, otherDataCenters, dc_err := topo.RandomlyPickNodes(rp.DiffDataCenterCount+1, func(node topology.Node) bool {
+		if preferredDataCenter != "" && node.IsDataCenter() && node.Id() != topology.NodeId(preferredDataCenter) {
+			return false
 		}
-	case storage.Copy110:
-		for i := 0; i < count; i++ {
-			nl := topology.NewNodeList(topo.Children(), nil)
-			picked, ret := nl.RandomlyPickN(2, 2, dataCenter)
-			vid := topo.NextVolumeId()
-			if ret {
-				var servers []*topology.DataNode
-				dc1, dc2 := picked[0], picked[1]
-				if dc2.FreeSpace() > dc1.FreeSpace() {
-					dc1, dc2 = dc2, dc1
-				}
-				if dc1.FreeSpace() > 0 {
-					if ok, server1 := dc1.ReserveOneVolume(rand.Intn(dc1.FreeSpace()), vid, ""); ok {
-						servers = append(servers, server1)
-						rack := server1.Parent()
-						exclusion := make(map[string]topology.Node)
-						exclusion[rack.String()] = rack
-						newNodeList := topology.NewNodeList(dc1.Children(), exclusion)
-						if newNodeList.FreeSpace() > 0 {
-							if ok2, server2 := newNodeList.ReserveOneVolume(rand.Intn(newNodeList.FreeSpace()), vid); ok2 {
-								servers = append(servers, server2)
-							}
-						}
-					}
-				}
-				if dc2.FreeSpace() > 0 {
-					if ok, server := dc2.ReserveOneVolume(rand.Intn(dc2.FreeSpace()), vid, ""); ok {
-						servers = append(servers, server)
-					}
-				}
-				if len(servers) == 3 {
-					if err = vg.grow(topo, vid, collection, repType, servers...); err == nil {
-						counter++
-					}
-				}
-			}
+		return node.FreeSpace() > rp.DiffRackCount+rp.SameRackCount+1
+	})
+	if dc_err != nil {
+		return nil, dc_err
+	}
+
+	//find main rack and other racks
+	mainRack, otherRacks, rack_err := mainDataCenter.(*topology.DataCenter).RandomlyPickNodes(rp.DiffRackCount+1, func(node topology.Node) bool {
+		return node.FreeSpace() > rp.SameRackCount+1
+	})
+	if rack_err != nil {
+		return nil, rack_err
+	}
+
+	//find main rack and other racks
+	mainServer, otherServers, server_err := mainRack.(*topology.Rack).RandomlyPickNodes(rp.SameRackCount+1, func(node topology.Node) bool {
+		return node.FreeSpace() > 1
+	})
+	if server_err != nil {
+		return nil, server_err
+	}
+
+	servers = append(servers, mainServer.(*topology.DataNode))
+	for _, server := range otherServers {
+		servers = append(servers, server.(*topology.DataNode))
+	}
+	for _, rack := range otherRacks {
+		r := rand.Intn(rack.FreeSpace())
+		if server, e := rack.ReserveOneVolume(r); e == nil {
+			servers = append(servers, server)
+		} else {
+			return servers, e
 		}
-	case storage.Copy200:
-		for i := 0; i < count; i++ {
-			nl := topology.NewNodeList(topo.Children(), nil)
-			picked, ret := nl.RandomlyPickN(3, 1, dataCenter)
-			vid := topo.NextVolumeId()
-			if ret {
-				var servers []*topology.DataNode
-				for _, n := range picked {
-					if n.FreeSpace() > 0 {
-						if ok, server := n.ReserveOneVolume(rand.Intn(n.FreeSpace()), vid, ""); ok {
-							servers = append(servers, server)
-						}
-					}
-				}
-				if len(servers) == 3 {
-					if err = vg.grow(topo, vid, collection, repType, servers...); err == nil {
-						counter++
-					}
-				}
-			}
+	}
+	for _, datacenter := range otherDataCenters {
+		r := rand.Intn(datacenter.FreeSpace())
+		if server, e := datacenter.ReserveOneVolume(r); e == nil {
+			servers = append(servers, server)
+		} else {
+			return servers, e
 		}
 	}
 	return
 }
-func (vg *VolumeGrowth) grow(topo *topology.Topology, vid storage.VolumeId, collection string, repType storage.ReplicationType, servers ...*topology.DataNode) error {
+
+func (vg *VolumeGrowth) grow(topo *topology.Topology, vid storage.VolumeId, collection string, rp *storage.ReplicaPlacement, servers ...*topology.DataNode) error {
 	for _, server := range servers {
-		if err := AllocateVolume(server, vid, collection, repType); err == nil {
-			vi := storage.VolumeInfo{Id: vid, Size: 0, Collection: collection, RepType: repType, Version: storage.CurrentVersion}
+		if err := AllocateVolume(server, vid, collection, rp); err == nil {
+			vi := storage.VolumeInfo{Id: vid, Size: 0, Collection: collection, ReplicaPlacement: rp, Version: storage.CurrentVersion}
 			server.AddOrUpdateVolume(vi)
 			topo.RegisterVolumeLayout(&vi, server)
 			glog.V(0).Infoln("Created Volume", vid, "on", server)
diff --git a/go/replication/volume_growth_test.go b/go/replication/volume_growth_test.go
index 99f82a7fa..bb6cbe90e 100644
--- a/go/replication/volume_growth_test.go
+++ b/go/replication/volume_growth_test.go
@@ -13,7 +13,7 @@ var topologyLayout = `
 {
   "dc1":{
     "rack1":{
-      "server1":{
+      "server111":{
         "volumes":[
           {"id":1, "size":12312},
           {"id":2, "size":12312},
@@ -21,7 +21,7 @@ var topologyLayout = `
         ],
         "limit":3
       },
-      "server2":{
+      "server112":{
         "volumes":[
           {"id":4, "size":12312},
           {"id":5, "size":12312},
@@ -31,7 +31,7 @@ var topologyLayout = `
       }
     },
     "rack2":{
-      "server1":{
+      "server121":{
         "volumes":[
           {"id":4, "size":12312},
           {"id":5, "size":12312},
@@ -39,17 +39,17 @@ var topologyLayout = `
         ],
         "limit":4
       },
-      "server2":{
+      "server122":{
         "volumes":[],
         "limit":4
       },
-      "server3":{
+      "server123":{
         "volumes":[
           {"id":2, "size":12312},
           {"id":3, "size":12312},
           {"id":4, "size":12312}
         ],
-        "limit":2
+        "limit":5
       }
     }
   },
@@ -57,7 +57,7 @@ var topologyLayout = `
   },
   "dc3":{
     "rack2":{
-      "server1":{
+      "server321":{
         "volumes":[
           {"id":1, "size":12312},
           {"id":3, "size":12312},
@@ -113,14 +113,16 @@ func setup(topologyLayout string) *topology.Topology {
 	return topo
 }
 
-func TestRemoveDataCenter(t *testing.T) {
+func TestFindEmptySlotsForOneVolume(t *testing.T) {
 	topo := setup(topologyLayout)
-	topo.UnlinkChildNode(topology.NodeId("dc2"))
-	if topo.GetActiveVolumeCount() != 15 {
+	vg := NewDefaultVolumeGrowth()
+	rp, _ := storage.NewReplicaPlacementFromString("002")
+	servers, err := vg.findEmptySlotsForOneVolume(topo, "dc1", rp)
+	if err != nil {
+		fmt.Println("finding empty slots error :", err)
 		t.Fail()
 	}
-	topo.UnlinkChildNode(topology.NodeId("dc3"))
-	if topo.GetActiveVolumeCount() != 12 {
-		t.Fail()
+	for _, server := range servers {
+		fmt.Println("assigned node :", server.Id())
 	}
 }
diff --git a/go/storage/cdb_map.go b/go/storage/cdb_map.go
index 3e38b0bdb..0d790cc0f 100644
--- a/go/storage/cdb_map.go
+++ b/go/storage/cdb_map.go
@@ -76,8 +76,8 @@ func (m cdbMap) FileCount() int {
 func (m *cdbMap) DeletedCount() int {
 	return m.DeletionCounter
 }
-func (m *cdbMap) NextFileKey(count int) (uint64) {
-  return 0
+func (m *cdbMap) NextFileKey(count int) uint64 {
+	return 0
 }
 
 func getMetric(c *cdb.Cdb, m *mapMetric) error {
diff --git a/go/storage/compact_map_perf_test.go b/go/storage/compact_map_perf_test.go
index e940310c0..37b23a59f 100644
--- a/go/storage/compact_map_perf_test.go
+++ b/go/storage/compact_map_perf_test.go
@@ -3,8 +3,8 @@ package storage
 import (
 	"code.google.com/p/weed-fs/go/glog"
 	"code.google.com/p/weed-fs/go/util"
-	"os"
 	"log"
+	"os"
 	"testing"
 )
 
diff --git a/go/storage/replica_placement.go b/go/storage/replica_placement.go
new file mode 100644
index 000000000..55428749b
--- /dev/null
+++ b/go/storage/replica_placement.go
@@ -0,0 +1,61 @@
+package storage
+
+import (
+	"errors"
+	"fmt"
+)
+
+const (
+	ReplicaPlacementCount = 9
+)
+
+type ReplicaPlacement struct {
+	SameRackCount       int
+	DiffRackCount       int
+	DiffDataCenterCount int
+}
+
+func NewReplicaPlacementFromString(t string) (*ReplicaPlacement, error) {
+	rp := &ReplicaPlacement{}
+	for i, c := range t {
+		count := int(c - '0')
+		if 0 <= count && count <= 2 {
+			switch i {
+			case 0:
+				rp.DiffDataCenterCount = count
+			case 1:
+				rp.DiffRackCount = count
+			case 2:
+				rp.SameRackCount = count
+			}
+		} else {
+			return rp, errors.New("Unknown Replication Type:" + t)
+		}
+	}
+	return rp, nil
+}
+
+func NewReplicaPlacementFromByte(b byte) (*ReplicaPlacement, error) {
+	return NewReplicaPlacementFromString(fmt.Sprintf("%d", b))
+}
+
+func (rp *ReplicaPlacement) Byte() byte {
+	ret := rp.DiffDataCenterCount*100 + rp.DiffRackCount*10 + rp.SameRackCount
+	return byte(ret)
+}
+
+func (rp *ReplicaPlacement) String() string {
+	b := make([]byte, 3)
+	b[0] = byte(rp.DiffDataCenterCount + '0')
+	b[1] = byte(rp.DiffRackCount + '0')
+	b[2] = byte(rp.SameRackCount + '0')
+	return string(b)
+}
+
+func (rp *ReplicaPlacement) GetCopyCount() int {
+	return rp.DiffDataCenterCount + rp.DiffRackCount + rp.SameRackCount + 1
+}
+
+func (rp *ReplicaPlacement) GetReplicationLevelIndex() int {
+	return rp.DiffDataCenterCount*3 + rp.DiffRackCount*3 + rp.SameRackCount
+}
diff --git a/go/storage/replication_type.go b/go/storage/replication_type.go
deleted file mode 100644
index 0902d1016..000000000
--- a/go/storage/replication_type.go
+++ /dev/null
@@ -1,123 +0,0 @@
-package storage
-
-import (
-	"errors"
-)
-
-type ReplicationType string
-
-const (
-	Copy000              = ReplicationType("000") // single copy
-	Copy001              = ReplicationType("001") // 2 copies, both on the same racks,  and same data center
-	Copy010              = ReplicationType("010") // 2 copies, both on different racks, but same data center
-	Copy100              = ReplicationType("100") // 2 copies, each on different data center
-	Copy110              = ReplicationType("110") // 3 copies, 2 on different racks and local data center, 1 on different data center
-	Copy200              = ReplicationType("200") // 3 copies, each on dffereint data center
-	LengthRelicationType = 6
-	CopyNil              = ReplicationType(255) // nil value
-)
-
-func NewReplicationTypeFromString(t string) (ReplicationType, error) {
-	switch t {
-	case "000":
-		return Copy000, nil
-	case "001":
-		return Copy001, nil
-	case "010":
-		return Copy010, nil
-	case "100":
-		return Copy100, nil
-	case "110":
-		return Copy110, nil
-	case "200":
-		return Copy200, nil
-	}
-	return Copy000, errors.New("Unknown Replication Type:" + t)
-}
-func NewReplicationTypeFromByte(b byte) (ReplicationType, error) {
-	switch b {
-	case byte(000):
-		return Copy000, nil
-	case byte(001):
-		return Copy001, nil
-	case byte(010):
-		return Copy010, nil
-	case byte(100):
-		return Copy100, nil
-	case byte(110):
-		return Copy110, nil
-	case byte(200):
-		return Copy200, nil
-	}
-	return Copy000, errors.New("Unknown Replication Type:" + string(b))
-}
-
-func (r *ReplicationType) String() string {
-	switch *r {
-	case Copy000:
-		return "000"
-	case Copy001:
-		return "001"
-	case Copy010:
-		return "010"
-	case Copy100:
-		return "100"
-	case Copy110:
-		return "110"
-	case Copy200:
-		return "200"
-	}
-	return "000"
-}
-func (r *ReplicationType) Byte() byte {
-	switch *r {
-	case Copy000:
-		return byte(000)
-	case Copy001:
-		return byte(001)
-	case Copy010:
-		return byte(010)
-	case Copy100:
-		return byte(100)
-	case Copy110:
-		return byte(110)
-	case Copy200:
-		return byte(200)
-	}
-	return byte(000)
-}
-
-func (repType ReplicationType) GetReplicationLevelIndex() int {
-	switch repType {
-	case Copy000:
-		return 0
-	case Copy001:
-		return 1
-	case Copy010:
-		return 2
-	case Copy100:
-		return 3
-	case Copy110:
-		return 4
-	case Copy200:
-		return 5
-	}
-	return -1
-}
-func (repType ReplicationType) GetCopyCount() int {
-	switch repType {
-	case Copy000:
-		return 1
-	case Copy001:
-		return 2
-	case Copy010:
-		return 2
-	case Copy100:
-		return 2
-	case Copy110:
-		return 3
-	case Copy200:
-		return 3
-	}
-	return 0
-}
diff --git a/go/storage/store.go b/go/storage/store.go
index 52e78d27d..2df0e6cb7 100644
--- a/go/storage/store.go
+++ b/go/storage/store.go
@@ -79,8 +79,8 @@ func NewStore(port int, ip, publicUrl string, dirnames []string, maxVolumeCounts
 	}
 	return
 }
-func (s *Store) AddVolume(volumeListString string, collection string, replicationType string) error {
-	rt, e := NewReplicationTypeFromString(replicationType)
+func (s *Store) AddVolume(volumeListString string, collection string, replicaPlacement string) error {
+	rt, e := NewReplicaPlacementFromString(replicaPlacement)
 	if e != nil {
 		return e
 	}
@@ -130,13 +130,13 @@ func (s *Store) findFreeLocation() (ret *DiskLocation) {
 	}
 	return ret
 }
-func (s *Store) addVolume(vid VolumeId, collection string, replicationType ReplicationType) error {
+func (s *Store) addVolume(vid VolumeId, collection string, replicaPlacement *ReplicaPlacement) error {
 	if s.findVolume(vid) != nil {
 		return fmt.Errorf("Volume Id %s already exists!", vid)
 	}
 	if location := s.findFreeLocation(); location != nil {
-		glog.V(0).Infoln("In dir", location.directory, "adds volume =", vid, ", collection =", collection, ", replicationType =", replicationType)
-		if volume, err := NewVolume(location.directory, collection, vid, replicationType); err == nil {
+		glog.V(0).Infoln("In dir", location.directory, "adds volume =", vid, ", collection =", collection, ", replicaPlacement =", replicaPlacement)
+		if volume, err := NewVolume(location.directory, collection, vid, replicaPlacement); err == nil {
 			location.volumes[vid] = volume
 			return nil
 		} else {
@@ -206,9 +206,9 @@ func (l *DiskLocation) loadExistingVolumes() {
 				}
 				if vid, err := NewVolumeId(base); err == nil {
 					if l.volumes[vid] == nil {
-						if v, e := NewVolume(l.directory, collection, vid, CopyNil); e == nil {
+						if v, e := NewVolume(l.directory, collection, vid, nil); e == nil {
 							l.volumes[vid] = v
-							glog.V(0).Infoln("data file", l.directory+"/"+name, "replicationType =", v.ReplicaType, "version =", v.Version(), "size =", v.Size())
+							glog.V(0).Infoln("data file", l.directory+"/"+name, "replicaPlacement =", v.ReplicaPlacement, "version =", v.Version(), "size =", v.Size())
 						}
 					}
 				}
@@ -223,7 +223,7 @@ func (s *Store) Status() []*VolumeInfo {
 		for k, v := range location.volumes {
 			s := &VolumeInfo{Id: VolumeId(k), Size: v.ContentSize(),
 				Collection:       v.Collection,
-				RepType:          v.ReplicaType,
+				ReplicaPlacement: v.ReplicaPlacement,
 				Version:          v.Version(),
 				FileCount:        v.nm.FileCount(),
 				DeleteCount:      v.nm.DeletedCount(),
@@ -261,7 +261,7 @@ func (s *Store) Join() error {
 		for k, v := range location.volumes {
 			s := &VolumeInfo{Id: VolumeId(k), Size: uint64(v.Size()),
 				Collection:       v.Collection,
-				RepType:          v.ReplicaType,
+				ReplicaPlacement: v.ReplicaPlacement,
 				Version:          v.Version(),
 				FileCount:        v.nm.FileCount(),
 				DeleteCount:      v.nm.DeletedCount(),
diff --git a/go/storage/volume.go b/go/storage/volume.go
index a8d8f9a58..59c3055e3 100644
--- a/go/storage/volume.go
+++ b/go/storage/volume.go
@@ -17,14 +17,14 @@ const (
 )
 
 type SuperBlock struct {
-	Version     Version
-	ReplicaType ReplicationType
+	Version          Version
+	ReplicaPlacement *ReplicaPlacement
 }
 
 func (s *SuperBlock) Bytes() []byte {
 	header := make([]byte, SuperBlockSize)
 	header[0] = byte(s.Version)
-	header[1] = s.ReplicaType.Byte()
+	header[1] = s.ReplicaPlacement.Byte()
 	return header
 }
 
@@ -41,15 +41,15 @@ type Volume struct {
 	accessLock sync.Mutex
 }
 
-func NewVolume(dirname string, collection string, id VolumeId, replicationType ReplicationType) (v *Volume, e error) {
+func NewVolume(dirname string, collection string, id VolumeId, replicaPlacement *ReplicaPlacement) (v *Volume, e error) {
 	v = &Volume{dir: dirname, Collection: collection, Id: id}
-	v.SuperBlock = SuperBlock{ReplicaType: replicationType}
+	v.SuperBlock = SuperBlock{ReplicaPlacement: replicaPlacement}
 	e = v.load(true, true)
 	return
 }
 func loadVolumeWithoutIndex(dirname string, collection string, id VolumeId) (v *Volume, e error) {
 	v = &Volume{dir: dirname, Collection: collection, Id: id}
-	v.SuperBlock = SuperBlock{ReplicaType: CopyNil}
+	v.SuperBlock = SuperBlock{}
 	e = v.load(false, false)
 	return
 }
@@ -90,7 +90,7 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool) error {
 		}
 	}
 
-	if v.ReplicaType == CopyNil {
+	if v.ReplicaPlacement == nil {
 		e = v.readSuperBlock()
 	} else {
 		e = v.maybeWriteSuperBlock()
@@ -173,13 +173,13 @@ func (v *Volume) readSuperBlock() (err error) {
 }
 func ParseSuperBlock(header []byte) (superBlock SuperBlock, err error) {
 	superBlock.Version = Version(header[0])
-	if superBlock.ReplicaType, err = NewReplicationTypeFromByte(header[1]); err != nil {
+	if superBlock.ReplicaPlacement, err = NewReplicaPlacementFromByte(header[1]); err != nil {
 		err = fmt.Errorf("cannot read replica type: %s", err.Error())
 	}
 	return
 }
 func (v *Volume) NeedToReplicate() bool {
-	return v.ReplicaType.GetCopyCount() > 1
+	return v.ReplicaPlacement.GetCopyCount() > 1
 }
 
 func (v *Volume) isFileUnchanged(n *Needle) bool {
diff --git a/go/storage/volume_info.go b/go/storage/volume_info.go
index c8eb7612e..1dfb3dcae 100644
--- a/go/storage/volume_info.go
+++ b/go/storage/volume_info.go
@@ -5,7 +5,7 @@ import ()
 type VolumeInfo struct {
 	Id               VolumeId
 	Size             uint64
-	RepType          ReplicationType
+	ReplicaPlacement *ReplicaPlacement
 	Collection       string
 	Version          Version
 	FileCount        int
diff --git a/go/topology/collection.go b/go/topology/collection.go
index 0a7971424..8042369a9 100644
--- a/go/topology/collection.go
+++ b/go/topology/collection.go
@@ -13,17 +13,17 @@ type Collection struct {
 
 func NewCollection(name string, volumeSizeLimit uint64) *Collection {
 	c := &Collection{Name: name, volumeSizeLimit: volumeSizeLimit}
-	c.replicaType2VolumeLayout = make([]*VolumeLayout, storage.LengthRelicationType)
+	c.replicaType2VolumeLayout = make([]*VolumeLayout, storage.ReplicaPlacementCount)
 	return c
 }
 
-func (c *Collection) GetOrCreateVolumeLayout(repType storage.ReplicationType) *VolumeLayout {
-	replicationTypeIndex := repType.GetReplicationLevelIndex()
-	if c.replicaType2VolumeLayout[replicationTypeIndex] == nil {
-		glog.V(0).Infoln("collection", c.Name, "adding replication type", repType)
-		c.replicaType2VolumeLayout[replicationTypeIndex] = NewVolumeLayout(repType, c.volumeSizeLimit)
+func (c *Collection) GetOrCreateVolumeLayout(rp *storage.ReplicaPlacement) *VolumeLayout {
+	replicaPlacementIndex := rp.GetReplicationLevelIndex()
+	if c.replicaType2VolumeLayout[replicaPlacementIndex] == nil {
+		glog.V(0).Infoln("collection", c.Name, "adding replication type", rp)
+		c.replicaType2VolumeLayout[replicaPlacementIndex] = NewVolumeLayout(rp, c.volumeSizeLimit)
 	}
-	return c.replicaType2VolumeLayout[replicationTypeIndex]
+	return c.replicaType2VolumeLayout[replicaPlacementIndex]
 }
 
 func (c *Collection) Lookup(vid storage.VolumeId) []*DataNode {
diff --git a/go/topology/data_center.go b/go/topology/data_center.go
index a3b2b7d13..ebd07803b 100644
--- a/go/topology/data_center.go
+++ b/go/topology/data_center.go
@@ -29,6 +29,7 @@ func (dc *DataCenter) GetOrCreateRack(rackName string) *Rack {
 
 func (dc *DataCenter) ToMap() interface{} {
 	m := make(map[string]interface{})
+	m["Id"] = dc.Id()
 	m["Max"] = dc.GetMaxVolumeCount()
 	m["Free"] = dc.FreeSpace()
 	var racks []interface{}
diff --git a/go/topology/node.go b/go/topology/node.go
index cfd6f6489..abe363b39 100644
--- a/go/topology/node.go
+++ b/go/topology/node.go
@@ -3,6 +3,8 @@ package topology
 import (
 	"code.google.com/p/weed-fs/go/glog"
 	"code.google.com/p/weed-fs/go/storage"
+	"errors"
+	"math/rand"
 )
 
 type NodeId string
@@ -10,7 +12,7 @@ type Node interface {
 	Id() NodeId
 	String() string
 	FreeSpace() int
-	ReserveOneVolume(r int, vid storage.VolumeId, dataCenter string) (bool, *DataNode)
+	ReserveOneVolume(r int) (*DataNode, error)
 	UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int)
 	UpAdjustVolumeCountDelta(volumeCountDelta int)
 	UpAdjustActiveVolumeCountDelta(activeVolumeCountDelta int)
@@ -47,6 +49,54 @@ type NodeImpl struct {
 	value    interface{}
 }
 
+// the first node must satisfy filterFirstNodeFn(), the rest nodes must have one free slot
+func (n *NodeImpl) RandomlyPickNodes(numberOfNodes int, filterFirstNodeFn func(dn Node) bool) (firstNode Node, restNodes []Node, err error) {
+	candidates := make([]Node, 0, len(n.children))
+	for _, node := range n.children {
+		if filterFirstNodeFn(node) {
+			candidates = append(candidates, node)
+		}
+	}
+	if len(candidates) == 0 {
+		return nil, nil, errors.New("No matching data node found!")
+	}
+	firstNode = candidates[rand.Intn(len(candidates))]
+	glog.V(2).Infoln(n.Id(), "picked main node:", firstNode.Id())
+
+	restNodes = make([]Node, numberOfNodes-1)
+	candidates = candidates[:0]
+	for _, node := range n.children {
+		if node.Id() == firstNode.Id() {
+			continue
+		}
+		if node.FreeSpace() <= 0 {
+			continue
+		}
+		glog.V(2).Infoln("select rest node candidate:", node.Id())
+		candidates = append(candidates, node)
+	}
+	glog.V(2).Infoln(n.Id(), "picking", numberOfNodes-1, "from rest", len(candidates), "node candidates")
+	ret := len(restNodes) == 0
+	for k, node := range candidates {
+		if k < len(restNodes) {
+			restNodes[k] = node
+			if k == len(restNodes)-1 {
+				ret = true
+			}
+		} else {
+			r := rand.Intn(k + 1)
+			if r < len(restNodes) {
+				restNodes[r] = node
+			}
+		}
+	}
+	if !ret {
+		glog.V(2).Infoln(n.Id(), "failed to pick", numberOfNodes-1, "from rest", len(candidates), "node candidates")
+		err = errors.New("Not enough data node found!")
+	}
+	return
+}
+
 func (n *NodeImpl) IsDataNode() bool {
 	return n.nodeType == "DataNode"
 }
@@ -80,32 +130,27 @@ func (n *NodeImpl) Parent() Node {
 func (n *NodeImpl) GetValue() interface{} {
 	return n.value
 }
-func (n *NodeImpl) ReserveOneVolume(r int, vid storage.VolumeId, dataCenter string) (bool, *DataNode) {
-	ret := false
-	var assignedNode *DataNode
+func (n *NodeImpl) ReserveOneVolume(r int) (assignedNode *DataNode, err error) {
 	for _, node := range n.children {
 		freeSpace := node.FreeSpace()
 		// fmt.Println("r =", r, ", node =", node, ", freeSpace =", freeSpace)
 		if freeSpace <= 0 {
 			continue
 		}
-		if dataCenter != "" && node.IsDataCenter() && node.Id() != NodeId(dataCenter) {
-			continue
-		}
 		if r >= freeSpace {
 			r -= freeSpace
 		} else {
 			if node.IsDataNode() && node.FreeSpace() > 0 {
 				// fmt.Println("vid =", vid, " assigned to node =", node, ", freeSpace =", node.FreeSpace())
-				return true, node.(*DataNode)
+				return node.(*DataNode), nil
 			}
-			ret, assignedNode = node.ReserveOneVolume(r, vid, dataCenter)
-			if ret {
-				break
+			assignedNode, err = node.ReserveOneVolume(r)
+			if err != nil {
+				return
 			}
 		}
 	}
-	return ret, assignedNode
+	return
 }
 
 func (n *NodeImpl) UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int) { //can be negative
diff --git a/go/topology/node_list.go b/go/topology/node_list.go
deleted file mode 100644
index bed151b54..000000000
--- a/go/topology/node_list.go
+++ /dev/null
@@ -1,83 +0,0 @@
-package topology
-
-import (
-	"code.google.com/p/weed-fs/go/glog"
-	"code.google.com/p/weed-fs/go/storage"
-	"math/rand"
-)
-
-type NodeList struct {
-	nodes  map[NodeId]Node
-	except map[string]Node
-}
-
-func NewNodeList(nodes map[NodeId]Node, except map[string]Node) *NodeList {
-	m := make(map[NodeId]Node, len(nodes)-len(except))
-	for _, n := range nodes {
-		if except[n.String()] == nil {
-			m[n.Id()] = n
-		}
-	}
-	nl := &NodeList{nodes: m}
-	return nl
-}
-
-func (nl *NodeList) FreeSpace() int {
-	freeSpace := 0
-	for _, n := range nl.nodes {
-		freeSpace += n.FreeSpace()
-	}
-	return freeSpace
-}
-
-func (nl *NodeList) RandomlyPickN(count int, minSpace int, firstNodeName string) ([]Node, bool) {
-	var list []Node
-	var preferredNode *Node
-	if firstNodeName != "" {
-		for _, n := range nl.nodes {
-			if n.Id() == NodeId(firstNodeName) && n.FreeSpace() >= minSpace {
-				preferredNode = &n
-				break
-			}
-		}
-		if preferredNode == nil {
-			return list, false
-		}
-	}
-
-	for _, n := range nl.nodes {
-		if n.FreeSpace() >= minSpace && n.Id() != NodeId(firstNodeName) {
-			list = append(list, n)
-		}
-	}
-	if count > len(list) || count == len(list) && firstNodeName != "" {
-		return nil, false
-	}
-	for i := len(list); i > 0; i-- {
-		r := rand.Intn(i)
-		list[r], list[i-1] = list[i-1], list[r]
-	}
-	if firstNodeName != "" {
-		list[0] = *preferredNode
-	}
-	return list[:count], true
-}
-
-func (nl *NodeList) ReserveOneVolume(randomVolumeIndex int, vid storage.VolumeId) (bool, *DataNode) {
-	for _, node := range nl.nodes {
-		freeSpace := node.FreeSpace()
-		if randomVolumeIndex >= freeSpace {
-			randomVolumeIndex -= freeSpace
-		} else {
-			if node.IsDataNode() && node.FreeSpace() > 0 {
-				glog.V(0).Infoln("vid =", vid, " assigned to node =", node, ", freeSpace =", node.FreeSpace())
-				return true, node.(*DataNode)
-			}
-			children := node.Children()
-			newNodeList := NewNodeList(children, nl.except)
-			return newNodeList.ReserveOneVolume(randomVolumeIndex, vid)
-		}
-	}
-	return false, nil
-
-}
diff --git a/go/topology/node_list_test.go b/go/topology/node_list_test.go
deleted file mode 100644
index c526f55f8..000000000
--- a/go/topology/node_list_test.go
+++ /dev/null
@@ -1,60 +0,0 @@
-package topology
-
-import (
-	"code.google.com/p/weed-fs/go/sequence"
-	_ "fmt"
-	"strconv"
-	"testing"
-)
-
-func TestXYZ(t *testing.T) {
-	topo, err := NewTopology("topo", "/etc/weed.conf", sequence.NewMemorySequencer(), 234, 5)
-	if err != nil {
-		t.Error("cannot create new topology:", err)
-		t.FailNow()
-	}
-	for i := 0; i < 5; i++ {
-		dc := NewDataCenter("dc" + strconv.Itoa(i))
-		dc.activeVolumeCount = i
-		dc.maxVolumeCount = 5
-		topo.LinkChildNode(dc)
-	}
-	nl := NewNodeList(topo.Children(), nil)
-
-	picked, ret := nl.RandomlyPickN(1, 0, "")
-	if !ret || len(picked) != 1 {
-		t.Error("need to randomly pick 1 node")
-	}
-
-	picked, ret = nl.RandomlyPickN(1, 0, "dc1")
-	if !ret || len(picked) != 1 {
-		t.Error("need to randomly pick 1 node")
-	}
-	if picked[0].Id() != "dc1" {
-		t.Error("need to randomly pick 1 dc1 node")
-	}
-
-	picked, ret = nl.RandomlyPickN(2, 0, "dc1")
-	if !ret || len(picked) != 2 {
-		t.Error("need to randomly pick 1 node")
-	}
-	if picked[0].Id() != "dc1" {
-		t.Error("need to randomly pick 2 with one dc1 node")
-	}
-
-	picked, ret = nl.RandomlyPickN(4, 0, "")
-	if !ret || len(picked) != 4 {
-		t.Error("need to randomly pick 4 nodes")
-	}
-
-	picked, ret = nl.RandomlyPickN(5, 0, "")
-	if !ret || len(picked) != 5 {
-		t.Error("need to randomly pick 5 nodes")
-	}
-
-	picked, ret = nl.RandomlyPickN(6, 0, "")
-	if ret || len(picked) != 0 {
-		t.Error("can not randomly pick 6 nodes:", ret, picked)
-	}
-
-}
diff --git a/go/topology/rack.go b/go/topology/rack.go
index acc34417a..40e19dd0d 100644
--- a/go/topology/rack.go
+++ b/go/topology/rack.go
@@ -52,6 +52,7 @@ func (r *Rack) GetOrCreateDataNode(ip string, port int, publicUrl string, maxVol
 
 func (rack *Rack) ToMap() interface{} {
 	m := make(map[string]interface{})
+	m["Id"] = rack.Id()
 	m["Max"] = rack.GetMaxVolumeCount()
 	m["Free"] = rack.FreeSpace()
 	var dns []interface{}
diff --git a/go/topology/topo_test.go b/go/topology/topo_test.go
index c0edca7c1..f3ae2096b 100644
--- a/go/topology/topo_test.go
+++ b/go/topology/topo_test.go
@@ -5,9 +5,7 @@ import (
 	"code.google.com/p/weed-fs/go/storage"
 	"encoding/json"
 	"fmt"
-	"math/rand"
 	"testing"
-	"time"
 )
 
 var topologyLayout = `
@@ -124,15 +122,3 @@ func TestRemoveDataCenter(t *testing.T) {
 		t.Fail()
 	}
 }
-
-func TestReserveOneVolume(t *testing.T) {
-	topo := setup(topologyLayout)
-	rand.Seed(time.Now().UnixNano())
-	rand.Seed(1)
-	ret, node, vid := topo.RandomlyReserveOneVolume("dc1")
-	if node.Parent().Parent().Id() != NodeId("dc1") {
-		t.Fail()
-	}
-	fmt.Println("assigned :", ret, ", node :", node, ", volume id:", vid)
-
-}
diff --git a/go/topology/topology.go b/go/topology/topology.go
index 24b3ab337..1426f7a12 100644
--- a/go/topology/topology.go
+++ b/go/topology/topology.go
@@ -77,23 +77,13 @@ func (t *Topology) Lookup(collection string, vid storage.VolumeId) []*DataNode {
 	return nil
 }
 
-func (t *Topology) RandomlyReserveOneVolume(dataCenter string) (bool, *DataNode, *storage.VolumeId) {
-	if t.FreeSpace() <= 0 {
-		glog.V(0).Infoln("Topology does not have free space left!")
-		return false, nil, nil
-	}
-	vid := t.NextVolumeId()
-	ret, node := t.ReserveOneVolume(rand.Intn(t.FreeSpace()), vid, dataCenter)
-	return ret, node, &vid
-}
-
 func (t *Topology) NextVolumeId() storage.VolumeId {
 	vid := t.GetMaxVolumeId()
 	return vid.Next()
 }
 
-func (t *Topology) PickForWrite(collectionName string, repType storage.ReplicationType, count int, dataCenter string) (string, int, *DataNode, error) {
-	vid, count, datanodes, err := t.GetVolumeLayout(collectionName, repType).PickForWrite(count, dataCenter)
+func (t *Topology) PickForWrite(collectionName string, rp *storage.ReplicaPlacement, count int, dataCenter string) (string, int, *DataNode, error) {
+	vid, count, datanodes, err := t.GetVolumeLayout(collectionName, rp).PickForWrite(count, dataCenter)
 	if err != nil || datanodes.Length() == 0 {
 		return "", 0, nil, errors.New("No writable volumes avalable!")
 	}
@@ -101,16 +91,16 @@ func (t *Topology) PickForWrite(collectionName string, repType storage.Replicati
 	return storage.NewFileId(*vid, fileId, rand.Uint32()).String(), count, datanodes.Head(), nil
 }
 
-func (t *Topology) GetVolumeLayout(collectionName string, repType storage.ReplicationType) *VolumeLayout {
+func (t *Topology) GetVolumeLayout(collectionName string, rp *storage.ReplicaPlacement) *VolumeLayout {
 	_, ok := t.collectionMap[collectionName]
 	if !ok {
 		t.collectionMap[collectionName] = NewCollection(collectionName, t.volumeSizeLimit)
 	}
-	return t.collectionMap[collectionName].GetOrCreateVolumeLayout(repType)
+	return t.collectionMap[collectionName].GetOrCreateVolumeLayout(rp)
 }
 
 func (t *Topology) RegisterVolumeLayout(v *storage.VolumeInfo, dn *DataNode) {
-	t.GetVolumeLayout(v.Collection, v.RepType).RegisterVolume(v, dn)
+	t.GetVolumeLayout(v.Collection, v.ReplicaPlacement).RegisterVolume(v, dn)
 }
 
 func (t *Topology) RegisterVolumes(init bool, volumeInfos []storage.VolumeInfo, ip string, port int, publicUrl string, maxVolumeCount int, dcName string, rackName string) {
diff --git a/go/topology/topology_event_handling.go b/go/topology/topology_event_handling.go
index 5097e9874..5740c9a03 100644
--- a/go/topology/topology_event_handling.go
+++ b/go/topology/topology_event_handling.go
@@ -41,7 +41,7 @@ func (t *Topology) StartRefreshWritableVolumes(garbageThreshold string) {
 	}()
 }
 func (t *Topology) SetVolumeCapacityFull(volumeInfo storage.VolumeInfo) bool {
-	vl := t.GetVolumeLayout(volumeInfo.Collection, volumeInfo.RepType)
+	vl := t.GetVolumeLayout(volumeInfo.Collection, volumeInfo.ReplicaPlacement)
 	if !vl.SetVolumeCapacityFull(volumeInfo.Id) {
 		return false
 	}
@@ -53,7 +53,7 @@ func (t *Topology) SetVolumeCapacityFull(volumeInfo storage.VolumeInfo) bool {
 func (t *Topology) UnRegisterDataNode(dn *DataNode) {
 	for _, v := range dn.volumes {
 		glog.V(0).Infoln("Removing Volume", v.Id, "from the dead volume server", dn)
-		vl := t.GetVolumeLayout(v.Collection, v.RepType)
+		vl := t.GetVolumeLayout(v.Collection, v.ReplicaPlacement)
 		vl.SetVolumeUnavailable(dn, v.Id)
 	}
 	dn.UpAdjustVolumeCountDelta(-dn.GetVolumeCount())
@@ -63,7 +63,7 @@ func (t *Topology) UnRegisterDataNode(dn *DataNode) {
 }
 func (t *Topology) RegisterRecoveredDataNode(dn *DataNode) {
 	for _, v := range dn.volumes {
-		vl := t.GetVolumeLayout(v.Collection, v.RepType)
+		vl := t.GetVolumeLayout(v.Collection, v.ReplicaPlacement)
 		if vl.isWritable(&v) {
 			vl.SetVolumeAvailable(dn, v.Id)
 		}
diff --git a/go/topology/volume_layout.go b/go/topology/volume_layout.go
index 8877d7ccf..40628b4a0 100644
--- a/go/topology/volume_layout.go
+++ b/go/topology/volume_layout.go
@@ -9,16 +9,16 @@ import (
 )
 
 type VolumeLayout struct {
-	repType         storage.ReplicationType
+	rp              *storage.ReplicaPlacement
 	vid2location    map[storage.VolumeId]*VolumeLocationList
 	writables       []storage.VolumeId // transient array of writable volume id
 	volumeSizeLimit uint64
 	accessLock      sync.Mutex
 }
 
-func NewVolumeLayout(repType storage.ReplicationType, volumeSizeLimit uint64) *VolumeLayout {
+func NewVolumeLayout(rp *storage.ReplicaPlacement, volumeSizeLimit uint64) *VolumeLayout {
 	return &VolumeLayout{
-		repType:         repType,
+		rp:              rp,
 		vid2location:    make(map[storage.VolumeId]*VolumeLocationList),
 		writables:       *new([]storage.VolumeId),
 		volumeSizeLimit: volumeSizeLimit,
@@ -33,7 +33,7 @@ func (vl *VolumeLayout) RegisterVolume(v *storage.VolumeInfo, dn *DataNode) {
 		vl.vid2location[v.Id] = NewVolumeLocationList()
 	}
 	if vl.vid2location[v.Id].Add(dn) {
-		if len(vl.vid2location[v.Id].list) == v.RepType.GetCopyCount() {
+		if len(vl.vid2location[v.Id].list) == v.ReplicaPlacement.GetCopyCount() {
 			if vl.isWritable(v) {
 				vl.writables = append(vl.writables, v.Id)
 			} else {
@@ -135,8 +135,8 @@ func (vl *VolumeLayout) SetVolumeUnavailable(dn *DataNode, vid storage.VolumeId)
 	defer vl.accessLock.Unlock()
 
 	if vl.vid2location[vid].Remove(dn) {
-		if vl.vid2location[vid].Length() < vl.repType.GetCopyCount() {
-			glog.V(0).Infoln("Volume", vid, "has", vl.vid2location[vid].Length(), "replica, less than required", vl.repType.GetCopyCount())
+		if vl.vid2location[vid].Length() < vl.rp.GetCopyCount() {
+			glog.V(0).Infoln("Volume", vid, "has", vl.vid2location[vid].Length(), "replica, less than required", vl.rp.GetCopyCount())
 			return vl.removeFromWritable(vid)
 		}
 	}
@@ -147,7 +147,7 @@ func (vl *VolumeLayout) SetVolumeAvailable(dn *DataNode, vid storage.VolumeId) b
 	defer vl.accessLock.Unlock()
 
 	if vl.vid2location[vid].Add(dn) {
-		if vl.vid2location[vid].Length() >= vl.repType.GetCopyCount() {
+		if vl.vid2location[vid].Length() >= vl.rp.GetCopyCount() {
 			return vl.setVolumeWritable(vid)
 		}
 	}
@@ -164,7 +164,7 @@ func (vl *VolumeLayout) SetVolumeCapacityFull(vid storage.VolumeId) bool {
 
 func (vl *VolumeLayout) ToMap() map[string]interface{} {
 	m := make(map[string]interface{})
-	m["replication"] = vl.repType.String()
+	m["replication"] = vl.rp.String()
 	m["writables"] = vl.writables
 	//m["locations"] = vl.vid2location
 	return m
diff --git a/go/util/file_util.go b/go/util/file_util.go
index 46e404851..8444296d3 100644
--- a/go/util/file_util.go
+++ b/go/util/file_util.go
@@ -1,7 +1,7 @@
 package util
 
 import (
-  "code.google.com/p/weed-fs/go/glog"
+	"code.google.com/p/weed-fs/go/glog"
 	"errors"
 	"os"
 )
diff --git a/go/weed/compact.go b/go/weed/compact.go
index 2600b3362..580f3f98d 100644
--- a/go/weed/compact.go
+++ b/go/weed/compact.go
@@ -33,7 +33,7 @@ func runCompact(cmd *Command, args []string) bool {
 	}
 
 	vid := storage.VolumeId(*compactVolumeId)
-	v, err := storage.NewVolume(*compactVolumePath, *compactVolumeCollection, vid, storage.CopyNil)
+	v, err := storage.NewVolume(*compactVolumePath, *compactVolumeCollection, vid, nil)
 	if err != nil {
 		glog.Fatalf("Load Volume [ERROR] %s\n", err)
 	}
diff --git a/go/weed/download.go b/go/weed/download.go
index 4f332bd2e..2e32f3ae9 100644
--- a/go/weed/download.go
+++ b/go/weed/download.go
@@ -49,7 +49,7 @@ func runDownload(cmd *Command, args []string) bool {
 			filename = fid
 		}
 		if strings.HasSuffix(filename, "-list") {
-		  filename = filename[0:len(filename)-len("-list")]
+			filename = filename[0 : len(filename)-len("-list")]
 			fids := strings.Split(string(content), "\n")
 			f, err := os.OpenFile(path.Join(*downloadDir, filename), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, os.ModePerm)
 			if err != nil {
diff --git a/go/weed/export.go b/go/weed/export.go
index 2ab197652..02452950d 100644
--- a/go/weed/export.go
+++ b/go/weed/export.go
@@ -82,8 +82,8 @@ func runExport(cmd *Command, args []string) bool {
 	}
 
 	fileName := strconv.Itoa(*exportVolumeId)
-	if *exportCollection!=""{
-	  fileName = *exportCollection + "_" + fileName
+	if *exportCollection != "" {
+		fileName = *exportCollection + "_" + fileName
 	}
 	vid := storage.VolumeId(*exportVolumeId)
 	indexFile, err := os.OpenFile(path.Join(*exportVolumePath, fileName+".idx"), os.O_RDONLY, 0644)
diff --git a/go/weed/master.go b/go/weed/master.go
index 97def1948..0d2866c68 100644
--- a/go/weed/master.go
+++ b/go/weed/master.go
@@ -27,18 +27,18 @@ var cmdMaster = &Command{
 }
 
 var (
-	mport                 = cmdMaster.Flag.Int("port", 9333, "http listen port")
-	masterIp              = cmdMaster.Flag.String("ip", "", "master ip address")
-	metaFolder            = cmdMaster.Flag.String("mdir", os.TempDir(), "data directory to store meta data")
-	masterPeers           = cmdMaster.Flag.String("peers", "", "other master nodes in comma separated ip:port list")
-	volumeSizeLimitMB     = cmdMaster.Flag.Uint("volumeSizeLimitMB", 32*1000, "Default Volume Size in MegaBytes")
-	mpulse                = cmdMaster.Flag.Int("pulseSeconds", 5, "number of seconds between heartbeats")
-	confFile              = cmdMaster.Flag.String("conf", "/etc/weedfs/weedfs.conf", "xml configuration file")
-	defaultRepType        = cmdMaster.Flag.String("defaultReplicationType", "000", "Default replication type if not specified.")
-	mReadTimeout          = cmdMaster.Flag.Int("readTimeout", 3, "connection read timeout in seconds")
-	mMaxCpu               = cmdMaster.Flag.Int("maxCpu", 0, "maximum number of CPUs. 0 means all available CPUs")
-	garbageThreshold      = cmdMaster.Flag.String("garbageThreshold", "0.3", "threshold to vacuum and reclaim spaces")
-	masterWhiteListOption = cmdMaster.Flag.String("whiteList", "", "comma separated Ip addresses having write permission. No limit if empty.")
+	mport                   = cmdMaster.Flag.Int("port", 9333, "http listen port")
+	masterIp                = cmdMaster.Flag.String("ip", "", "master ip address")
+	metaFolder              = cmdMaster.Flag.String("mdir", os.TempDir(), "data directory to store meta data")
+	masterPeers             = cmdMaster.Flag.String("peers", "", "other master nodes in comma separated ip:port list")
+	volumeSizeLimitMB       = cmdMaster.Flag.Uint("volumeSizeLimitMB", 32*1000, "Default Volume Size in MegaBytes")
+	mpulse                  = cmdMaster.Flag.Int("pulseSeconds", 5, "number of seconds between heartbeats")
+	confFile                = cmdMaster.Flag.String("conf", "/etc/weedfs/weedfs.conf", "xml configuration file")
+	defaultReplicaPlacement = cmdMaster.Flag.String("defaultReplication", "000", "Default replication type if not specified.")
+	mReadTimeout            = cmdMaster.Flag.Int("readTimeout", 3, "connection read timeout in seconds")
+	mMaxCpu                 = cmdMaster.Flag.Int("maxCpu", 0, "maximum number of CPUs. 0 means all available CPUs")
+	garbageThreshold        = cmdMaster.Flag.String("garbageThreshold", "0.3", "threshold to vacuum and reclaim spaces")
+	masterWhiteListOption   = cmdMaster.Flag.String("whiteList", "", "comma separated Ip addresses having write permission. No limit if empty.")
 
 	masterWhiteList []string
 )
@@ -57,7 +57,7 @@ func runMaster(cmd *Command, args []string) bool {
 
 	r := mux.NewRouter()
 	ms := weed_server.NewMasterServer(r, VERSION, *mport, *metaFolder,
-		*volumeSizeLimitMB, *mpulse, *confFile, *defaultRepType, *garbageThreshold, masterWhiteList,
+		*volumeSizeLimitMB, *mpulse, *confFile, *defaultReplicaPlacement, *garbageThreshold, masterWhiteList,
 	)
 
 	glog.V(0).Infoln("Start Weed Master", VERSION, "at port", *masterIp+":"+strconv.Itoa(*mport))
diff --git a/go/weed/server.go b/go/weed/server.go
index b1f5dc049..4cd800142 100644
--- a/go/weed/server.go
+++ b/go/weed/server.go
@@ -35,23 +35,23 @@ var cmdServer = &Command{
 }
 
 var (
-	serverIp                  = cmdServer.Flag.String("ip", "localhost", "ip or server name")
-	serverMaxCpu              = cmdServer.Flag.Int("maxCpu", 0, "maximum number of CPUs. 0 means all available CPUs")
-	serverReadTimeout         = cmdServer.Flag.Int("readTimeout", 3, "connection read timeout in seconds. Increase this if uploading large files.")
-	serverDataCenter          = cmdServer.Flag.String("dataCenter", "", "current volume server's data center name")
-	serverRack                = cmdServer.Flag.String("rack", "", "current volume server's rack name")
-	serverWhiteListOption     = cmdServer.Flag.String("whiteList", "", "comma separated Ip addresses having write permission. No limit if empty.")
-	serverPeers               = cmdServer.Flag.String("peers", "", "other master nodes in comma separated ip:masterPort list")
-	masterPort                = cmdServer.Flag.Int("masterPort", 9333, "master server http listen port")
-	masterMetaFolder          = cmdServer.Flag.String("mdir", os.TempDir(), "data directory to store meta data")
-	masterVolumeSizeLimitMB   = cmdServer.Flag.Uint("volumeSizeLimitMB", 32*1000, "Default Volume Size in MegaBytes")
-	masterConfFile            = cmdServer.Flag.String("conf", "/etc/weedfs/weedfs.conf", "xml configuration file")
-	masterDefaultRepType      = cmdServer.Flag.String("defaultReplicationType", "000", "Default replication type if not specified.")
-	volumePort                = cmdServer.Flag.Int("port", 8080, "volume server http listen port")
-	volumePublicUrl           = cmdServer.Flag.String("publicUrl", "", "Publicly accessible <ip|server_name>:<port>")
-	volumeDataFolders         = cmdServer.Flag.String("dir", os.TempDir(), "directories to store data files. dir[,dir]...")
-	volumeMaxDataVolumeCounts = cmdServer.Flag.String("max", "7", "maximum numbers of volumes, count[,count]...")
-	volumePulse               = cmdServer.Flag.Int("pulseSeconds", 5, "number of seconds between heartbeats, must be smaller than the master's setting")
+	serverIp                      = cmdServer.Flag.String("ip", "localhost", "ip or server name")
+	serverMaxCpu                  = cmdServer.Flag.Int("maxCpu", 0, "maximum number of CPUs. 0 means all available CPUs")
+	serverReadTimeout             = cmdServer.Flag.Int("readTimeout", 3, "connection read timeout in seconds. Increase this if uploading large files.")
+	serverDataCenter              = cmdServer.Flag.String("dataCenter", "", "current volume server's data center name")
+	serverRack                    = cmdServer.Flag.String("rack", "", "current volume server's rack name")
+	serverWhiteListOption         = cmdServer.Flag.String("whiteList", "", "comma separated Ip addresses having write permission. No limit if empty.")
+	serverPeers                   = cmdServer.Flag.String("peers", "", "other master nodes in comma separated ip:masterPort list")
+	masterPort                    = cmdServer.Flag.Int("masterPort", 9333, "master server http listen port")
+	masterMetaFolder              = cmdServer.Flag.String("mdir", "", "data directory to store meta data, default to same as -dir specified")
+	masterVolumeSizeLimitMB       = cmdServer.Flag.Uint("volumeSizeLimitMB", 32*1000, "Default Volume Size in MegaBytes")
+	masterConfFile                = cmdServer.Flag.String("conf", "/etc/weedfs/weedfs.conf", "xml configuration file")
+	masterDefaultReplicaPlacement = cmdServer.Flag.String("defaultReplicaPlacement", "000", "Default replication type if not specified.")
+	volumePort                    = cmdServer.Flag.Int("port", 8080, "volume server http listen port")
+	volumePublicUrl               = cmdServer.Flag.String("publicUrl", "", "Publicly accessible <ip|server_name>:<port>")
+	volumeDataFolders             = cmdServer.Flag.String("dir", os.TempDir(), "directories to store data files. dir[,dir]...")
+	volumeMaxDataVolumeCounts     = cmdServer.Flag.String("max", "7", "maximum numbers of volumes, count[,count]...")
+	volumePulse                   = cmdServer.Flag.Int("pulseSeconds", 5, "number of seconds between heartbeats, must be smaller than the master's setting")
 
 	serverWhiteList []string
 )
@@ -62,6 +62,9 @@ func runServer(cmd *Command, args []string) bool {
 	}
 	runtime.GOMAXPROCS(*serverMaxCpu)
 
+	if *masterMetaFolder == "" {
+		*masterMetaFolder = *volumeDataFolders
+	}
 	if err := util.TestFolderWritable(*masterMetaFolder); err != nil {
 		glog.Fatalf("Check Meta Folder (-mdir) Writable %s : %s", *masterMetaFolder, err)
 	}
@@ -95,7 +98,7 @@ func runServer(cmd *Command, args []string) bool {
 	go func() {
 		r := mux.NewRouter()
 		ms := weed_server.NewMasterServer(r, VERSION, *masterPort, *masterMetaFolder,
-			*masterVolumeSizeLimitMB, *volumePulse, *masterConfFile, *masterDefaultRepType, *garbageThreshold, serverWhiteList,
+			*masterVolumeSizeLimitMB, *volumePulse, *masterConfFile, *masterDefaultReplicaPlacement, *garbageThreshold, serverWhiteList,
 		)
 
 		glog.V(0).Infoln("Start Weed Master", VERSION, "at port", *serverIp+":"+strconv.Itoa(*masterPort))
diff --git a/go/weed/version.go b/go/weed/version.go
index 561057778..9b6ca63e1 100644
--- a/go/weed/version.go
+++ b/go/weed/version.go
@@ -6,7 +6,7 @@ import (
 )
 
 const (
-	VERSION = "0.46"
+	VERSION = "0.47"
 )
 
 var cmdVersion = &Command{
diff --git a/go/weed/weed_server/master_server.go b/go/weed/weed_server/master_server.go
index 738484ff0..c8a9bbbed 100644
--- a/go/weed/weed_server/master_server.go
+++ b/go/weed/weed_server/master_server.go
@@ -15,14 +15,14 @@ import (
 )
 
 type MasterServer struct {
-	port              int
-	metaFolder        string
-	volumeSizeLimitMB uint
-	pulseSeconds      int
-	defaultRepType    string
-	garbageThreshold  string
-	whiteList         []string
-	version           string
+	port                    int
+	metaFolder              string
+	volumeSizeLimitMB       uint
+	pulseSeconds            int
+	defaultReplicaPlacement string
+	garbageThreshold        string
+	whiteList               []string
+	version                 string
 
 	topo   *topology.Topology
 	vg     *replication.VolumeGrowth
@@ -35,17 +35,17 @@ func NewMasterServer(r *mux.Router, version string, port int, metaFolder string,
 	volumeSizeLimitMB uint,
 	pulseSeconds int,
 	confFile string,
-	defaultRepType string,
+	defaultReplicaPlacement string,
 	garbageThreshold string,
 	whiteList []string,
 ) *MasterServer {
 	ms := &MasterServer{
-		version:           version,
-		volumeSizeLimitMB: volumeSizeLimitMB,
-		pulseSeconds:      pulseSeconds,
-		defaultRepType:    defaultRepType,
-		garbageThreshold:  garbageThreshold,
-		whiteList:         whiteList,
+		version:                 version,
+		volumeSizeLimitMB:       volumeSizeLimitMB,
+		pulseSeconds:            pulseSeconds,
+		defaultReplicaPlacement: defaultReplicaPlacement,
+		garbageThreshold:        garbageThreshold,
+		whiteList:               whiteList,
 	}
 	seq := sequence.NewFileSequencer(path.Join(metaFolder, "weed.seq"))
 	var e error
diff --git a/go/weed/weed_server/master_server_handlers.go b/go/weed/weed_server/master_server_handlers.go
index 99e8fd1ef..f53d2b8ed 100644
--- a/go/weed/weed_server/master_server_handlers.go
+++ b/go/weed/weed_server/master_server_handlers.go
@@ -40,20 +40,20 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 	if e != nil {
 		c = 1
 	}
-	repType := r.FormValue("replication")
-	if repType == "" {
-		repType = ms.defaultRepType
+	replication := r.FormValue("replication")
+	if replication == "" {
+		replication = ms.defaultReplicaPlacement
 	}
 	collection := r.FormValue("collection")
 	dataCenter := r.FormValue("dataCenter")
-	rt, err := storage.NewReplicationTypeFromString(repType)
+	replicaPlacement, err := storage.NewReplicaPlacementFromString(replication)
 	if err != nil {
 		w.WriteHeader(http.StatusNotAcceptable)
 		writeJsonQuiet(w, r, map[string]string{"error": err.Error()})
 		return
 	}
 
-	if ms.topo.GetVolumeLayout(collection, rt).GetActiveVolumeCount(dataCenter) <= 0 {
+	if ms.topo.GetVolumeLayout(collection, replicaPlacement).GetActiveVolumeCount(dataCenter) <= 0 {
 		if ms.topo.FreeSpace() <= 0 {
 			w.WriteHeader(http.StatusNotFound)
 			writeJsonQuiet(w, r, map[string]string{"error": "No free volumes left!"})
@@ -61,15 +61,15 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 		} else {
 			ms.vgLock.Lock()
 			defer ms.vgLock.Unlock()
-			if ms.topo.GetVolumeLayout(collection, rt).GetActiveVolumeCount(dataCenter) <= 0 {
-				if _, err = ms.vg.AutomaticGrowByType(collection, rt, dataCenter, ms.topo); err != nil {
+			if ms.topo.GetVolumeLayout(collection, replicaPlacement).GetActiveVolumeCount(dataCenter) <= 0 {
+				if _, err = ms.vg.AutomaticGrowByType(collection, replicaPlacement, dataCenter, ms.topo); err != nil {
 					writeJsonQuiet(w, r, map[string]string{"error": "Cannot grow volume group! " + err.Error()})
 					return
 				}
 			}
 		}
 	}
-	fid, count, dn, err := ms.topo.PickForWrite(collection, rt, c, dataCenter)
+	fid, count, dn, err := ms.topo.PickForWrite(collection, replicaPlacement, c, dataCenter)
 	if err == nil {
 		writeJsonQuiet(w, r, map[string]interface{}{"fid": fid, "url": dn.Url(), "publicUrl": dn.PublicUrl, "count": count})
 	} else {
@@ -119,13 +119,13 @@ func (ms *MasterServer) volumeVacuumHandler(w http.ResponseWriter, r *http.Reque
 
 func (ms *MasterServer) volumeGrowHandler(w http.ResponseWriter, r *http.Request) {
 	count := 0
-	rt, err := storage.NewReplicationTypeFromString(r.FormValue("replication"))
+	replicaPlacement, err := storage.NewReplicaPlacementFromString(r.FormValue("replication"))
 	if err == nil {
 		if count, err = strconv.Atoi(r.FormValue("count")); err == nil {
-			if ms.topo.FreeSpace() < count*rt.GetCopyCount() {
-				err = errors.New("Only " + strconv.Itoa(ms.topo.FreeSpace()) + " volumes left! Not enough for " + strconv.Itoa(count*rt.GetCopyCount()))
+			if ms.topo.FreeSpace() < count*replicaPlacement.GetCopyCount() {
+				err = errors.New("Only " + strconv.Itoa(ms.topo.FreeSpace()) + " volumes left! Not enough for " + strconv.Itoa(count*replicaPlacement.GetCopyCount()))
 			} else {
-				count, err = ms.vg.GrowByCountAndType(count, r.FormValue("collection"), rt, r.FormValue("dataCneter"), ms.topo)
+				count, err = ms.vg.GrowByCountAndType(count, r.FormValue("collection"), replicaPlacement, r.FormValue("dataCneter"), ms.topo)
 			}
 		} else {
 			err = errors.New("parameter count is not found")
diff --git a/go/weed/weed_server/volume_server.go b/go/weed/weed_server/volume_server.go
index f42585da2..a2939f848 100644
--- a/go/weed/weed_server/volume_server.go
+++ b/go/weed/weed_server/volume_server.go
@@ -61,7 +61,7 @@ func NewVolumeServer(r *http.ServeMux, version string, ip string, port int, publ
 			if connected {
 				time.Sleep(time.Duration(float32(vs.pulseSeconds*1e3)*(1+rand.Float32())) * time.Millisecond)
 			} else {
-				time.Sleep(time.Duration(float32(vs.pulseSeconds*1e3)* 0.25) * time.Millisecond)
+				time.Sleep(time.Duration(float32(vs.pulseSeconds*1e3)*0.25) * time.Millisecond)
 			}
 		}
 	}()
diff --git a/note/replication.txt b/note/replication.txt
index c4bf46044..a151e80c3 100644
--- a/note/replication.txt
+++ b/note/replication.txt
@@ -59,11 +59,6 @@ If any "assign" request comes in
   3. return a writable volume to the user
 
   
-Plan:
-  Step 1. implement one copy(no replication), automatically assign volume ids
-  Step 2. add replication
-
-For the above operations, here are the todo list:
   for data node:
     0. detect existing volumes DONE
     1. onStartUp, and periodically, send existing volumes and maxVolumeCount  store.Join(), DONE
@@ -77,10 +72,38 @@ For the above operations, here are the todo list:
     1. accept data node's report of existing volumes and maxVolumeCount ALREADY EXISTS /dir/join
     2. periodically refresh for active data nodes, and adjust writable volumes
     3. send command to grow a volume(id + replication level)  DONE
-    4. NOT_IMPLEMENTING: if dead/stale data nodes are found, for the affected volumes, send stale info
-       to other data nodes. BECAUSE the master will stop sending writes to these data nodes
     5. accept lookup for volume locations    ALREADY EXISTS /dir/lookup
     6. read topology/datacenter/rack layout
 
+An algorithm to allocate volumes evenly, but may be inefficient if free volumes are plenty:
+input: replication=xyz
+algorithm:
+ret_dcs = []
+foreach dc that has y+z+1 volumes{
+  ret_racks = []
+  foreach rack with z+1 volumes{
+    ret = select z+1 servers with 1 volume
+  if ret.size()==z+1 {
+    ret_racks.append(ret)
+  }
+  }
+  randomly pick one rack from ret_racks
+  ret += select y racks with 1 volume each
+  if ret.size()==y+z+1{
+    ret_dcs.append(ret)
+  }
+}
+randomly pick one dc from ret_dcs
+ret += select x data centers with 1 volume each
+
+A simple replica placement algorithm, but may fail when free volume slots are not plenty:
+ret := []volumes
+dc = randomly pick 1 data center with y+z+1 volumes
+  rack = randomly pick 1 rack with z+1 volumes
+    ret = ret.append(randomly pick z+1 volumes)
+  ret = ret.append(randomly pick y racks with 1 volume)
+ret = ret.append(randomly pick x data centers with 1 volume)
+
+
 TODO:
   1. replicate content to the other server if the replication type needs replicas