diff --git a/weed-fs/note/startup_process.txt b/weed-fs/note/startup_process.txt
new file mode 100644
index 000000000..558955005
--- /dev/null
+++ b/weed-fs/note/startup_process.txt
@@ -0,0 +1,7 @@
+1. clients report its own server info, volumes info, 
+2. master collect all volumes, separated into readable volumes, writable volumes, volume2machine mapping
+  machines is an array of machine info
+  writable volumes is an array of vids
+  vid2machineId maps volume id to machineId, which is the index of machines array
+
+
diff --git a/weed-fs/src/cmd/weeds.go b/weed-fs/src/cmd/weeds.go
index b888c6c29..c639d48d3 100644
--- a/weed-fs/src/cmd/weeds.go
+++ b/weed-fs/src/cmd/weeds.go
@@ -8,7 +8,6 @@ import (
 	"http"
 	"json"
 	"log"
-	"rand"
 	"strconv"
 	"strings"
 )
@@ -22,20 +21,8 @@ var (
 
 func dirReadHandler(w http.ResponseWriter, r *http.Request) {
 	volumeId, _ := strconv.Atoi(r.FormValue("volumeId"))
-	machineList := mapper.Get(volumeId)
-	x := rand.Intn(len(machineList))
-	machine := machineList[x]
-	bytes, _ := json.Marshal(machine)
-	callback := r.FormValue("callback")
-	w.Header().Set("Content-Type", "application/javascript")
-	if callback == "" {
-		w.Write(bytes)
-	} else {
-		w.Write([]uint8(callback))
-		w.Write([]uint8("("))
-		w.Write(bytes)
-		w.Write([]uint8(")"))
-	}
+	machine := mapper.Get(volumeId)
+	writeJson(w, r, machine)
 }
 func dirWriteHandler(w http.ResponseWriter, r *http.Request) {
 	machineList := mapper.PickForWrite()
@@ -44,17 +31,17 @@ func dirWriteHandler(w http.ResponseWriter, r *http.Request) {
 func dirJoinHandler(w http.ResponseWriter, r *http.Request) {
 	s := r.RemoteAddr[0:strings.Index(r.RemoteAddr, ":")+1] + r.FormValue("port")
 	publicServer := r.FormValue("publicServer")
-	volumes := new([]storage.VolumeStat)
+	volumes := new([]storage.VolumeInfo)
 	json.Unmarshal([]byte(r.FormValue("volumes")), volumes)
 	capacity, _ := strconv.Atoi(r.FormValue("capacity"))
 	log.Println("Recieved joining request from remote address", s, "capacity=", capacity, "volumes", r.FormValue("volumes"))
-	vids := mapper.Add(*directory.NewMachine(s, publicServer), *volumes, capacity)
+	vids := mapper.Add(*directory.NewMachine(s, publicServer, *volumes, capacity))
 	writeJson(w, r, vids)
 }
 func dirStatusHandler(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "text/plain")
 	bytes, _ := json.Marshal(mapper)
-	fmt.Fprint(w, bytes)
+	fmt.Fprint(w, string(bytes))
 }
 func writeJson(w http.ResponseWriter, r *http.Request, obj interface{}) {
 	w.Header().Set("Content-Type", "application/javascript")
@@ -65,7 +52,7 @@ func writeJson(w http.ResponseWriter, r *http.Request, obj interface{}) {
 	} else {
 		w.Write([]uint8(callback))
 		w.Write([]uint8("("))
-		w.Write(bytes)
+		fmt.Fprint(w, string(bytes))
 		w.Write([]uint8(")"))
 	}
 	log.Println("JSON Response", string(bytes))
diff --git a/weed-fs/src/pkg/directory/volume_mapping.go b/weed-fs/src/pkg/directory/volume_mapping.go
index e9a0647e5..4062b3109 100644
--- a/weed-fs/src/pkg/directory/volume_mapping.go
+++ b/weed-fs/src/pkg/directory/volume_mapping.go
@@ -7,24 +7,36 @@ import (
 	"rand"
 	"log"
 	"storage"
+	"sync"
+)
+
+const (
+	ChunkSizeLimit = 1 * 1024 * 1024 * 1024 //1G, can not be more than max(uint32)*8
 )
 
 type Machine struct {
 	Server       string //<server name/ip>[:port]
 	PublicServer string
-	CanWrite     bool
+	Volumes      []storage.VolumeInfo
+	Capacity     int
 }
+
 type Mapper struct {
 	dir      string
 	fileName string
 	capacity int
-	Machines [][]Machine //initial version only support one copy per machine
-	writers  [][]Machine // transient value to lookup writers fast
+
+	lock          sync.Mutex
+	Machines      []*Machine
+	vid2machineId map[uint64]int
+	writers       []int // transient array of writers volume id
+
+	GlobalVolumeSequence uint64
 }
 
-func NewMachine(server, publicServer string) (m *Machine) {
+func NewMachine(server, publicServer string, volumes []storage.VolumeInfo, capacity int) (m *Machine) {
 	m = new(Machine)
-	m.Server, m.PublicServer = server, publicServer
+	m.Server, m.PublicServer, m.Volumes, m.Capacity = server, publicServer, volumes, capacity
 	return
 }
 
@@ -33,74 +45,90 @@ func NewMapper(dirname string, filename string, capacity int) (m *Mapper) {
 	m.dir, m.fileName, m.capacity = dirname, filename, capacity
 	log.Println("Loading volume id to maching mapping:", path.Join(m.dir, m.fileName+".map"))
 	dataFile, e := os.OpenFile(path.Join(m.dir, m.fileName+".map"), os.O_RDONLY, 0644)
-	m.Machines = *new([][]Machine)
-	m.writers = *new([][]Machine)
+    m.vid2machineId = make(map[uint64]int)
+    m.writers = *new([]int)
 	if e != nil {
 		log.Println("Mapping File Read", e)
+		m.Machines = *new([]*Machine)
 	} else {
 		decoder := gob.NewDecoder(dataFile)
-        defer dataFile.Close()
+		defer dataFile.Close()
 		decoder.Decode(&m.Machines)
-		for _, list := range m.Machines {
-			//TODO: what if a list has mixed readers and writers? Now it's treated as readonly
-			allCanWrite := false
-			for _, entry := range list {
-				allCanWrite = allCanWrite && entry.CanWrite
-			}
-			if allCanWrite {
-				m.writers = append(m.writers, list)
+		decoder.Decode(&m.GlobalVolumeSequence)
+
+		//add to vid2machineId map, and writers array
+		for machine_index, machine := range m.Machines {
+			for _, v := range machine.Volumes {
+		        m.vid2machineId[v.Id] = machine_index
+				if v.Size < ChunkSizeLimit {
+					m.writers = append(m.writers, machine_index)
+				}
 			}
 		}
-        log.Println("Loaded mapping size", len(m.Machines))
+		log.Println("Loaded mapping size", len(m.Machines))
 	}
 	return
 }
-func (m *Mapper) PickForWrite() []Machine {
-	vid := rand.Intn(len(m.Machines))
-	return m.Machines[vid]
+func (m *Mapper) PickForWrite() *Machine {
+	vid := rand.Intn(len(m.writers))
+	return m.Machines[m.writers[vid]]
 }
-func (m *Mapper) Get(vid int) []Machine {
+func (m *Mapper) Get(vid int) *Machine {
 	return m.Machines[vid]
 }
-func (m *Mapper) Add(machine Machine, volumes []storage.VolumeStat, capacity int) []int {
-	log.Println("Adding existing", machine.Server, len(volumes), "volumes to dir", len(m.Machines))
-    log.Println("Adding      new ", machine.Server, capacity - len(volumes), "volumes to dir", len(m.Machines))
-	maxId := len(m.Machines)-1
-	for _, v := range volumes {
-		if maxId < int(v.Id) {
-			maxId = int(v.Id)
+func (m *Mapper) Add(machine Machine) []uint64 {
+	log.Println("Adding existing", machine.Server, len(machine.Volumes), "volumes to dir", len(m.Machines))
+	log.Println("Adding      new ", machine.Server, machine.Capacity-len(machine.Volumes), "volumes to dir", len(m.Machines))
+	//check existing machine, linearly
+	m.lock.Lock()
+	foundExistingMachineId := -1
+	for index, entry := range m.Machines {
+		if machine.Server == entry.Server {
+			foundExistingMachineId = index
+			break
 		}
 	}
-	for i := len(m.Machines); i <= maxId; i++ {
-		m.Machines = append(m.Machines, nil)
-	}
-    log.Println("Machine list now is", len(m.Machines))
-	for _, v := range volumes {
-		found := false
-		existing := m.Machines[v.Id]
-		for _, entry := range existing {
-			if machine.Server == entry.Server {
-				found = true
-				break
-			}
-		}
-		if !found {
-			m.Machines[v.Id] = append(existing, machine)
-			log.Println("Setting volume", v.Id, "to", machine.Server)
-		}
+	machineId := foundExistingMachineId
+	if machineId < 0 {
+		machineId = len(m.Machines)
+		m.Machines = append(m.Machines, &machine)
 	}
 
-    vids := new([]int)
-	for vid,i := len(m.Machines),len(volumes); i < capacity; i,vid=i+1,vid+1 {
-		list := new([]Machine)
-		*list = append(*list, machine)
-		m.Machines = append(m.Machines, *list)
+	//generate new volumes
+	vids := new([]uint64)
+	for vid, i := m.GlobalVolumeSequence, len(machine.Volumes); i < machine.Capacity; i, vid = i+1, vid+1 {
+		newVolume := *new(storage.VolumeInfo)
+		newVolume.Id, newVolume.Size = vid, 0
+		machine.Volumes = append(machine.Volumes, newVolume)
+		m.vid2machineId[vid] = machineId
 		log.Println("Adding volume", vid, "from", machine.Server)
 		*vids = append(*vids, vid)
+		m.GlobalVolumeSequence = vid + 1
 	}
 
 	m.Save()
-	log.Println("Dir size =>", len(m.Machines))
+	m.lock.Unlock()
+
+	//add to vid2machineId map, and writers array
+	for _, v := range machine.Volumes {
+		log.Println("Setting volume", v.Id, "to", machine.Server)
+		m.vid2machineId[v.Id] = machineId
+		if v.Size < ChunkSizeLimit {
+			m.writers = append(m.writers, machineId)
+		}
+	}
+	//setting writers, copy-on-write because of possible updating
+	var writers []int
+    for machine_index, machine_entry := range m.Machines {
+        for _, v := range machine_entry.Volumes {
+            if v.Size < ChunkSizeLimit {
+                writers = append(writers, machine_index)
+            }
+        }
+    }
+    m.writers = writers
+
+	log.Println("Machines:", len(m.Machines), "Volumes:", len(m.vid2machineId), "Writable:", len(m.writers))
 	return *vids
 }
 func (m *Mapper) Save() {
@@ -112,4 +140,5 @@ func (m *Mapper) Save() {
 	defer dataFile.Close()
 	encoder := gob.NewEncoder(dataFile)
 	encoder.Encode(m.Machines)
+	encoder.Encode(m.GlobalVolumeSequence)
 }
diff --git a/weed-fs/src/pkg/storage/store.go b/weed-fs/src/pkg/storage/store.go
index e13f47257..ff6944853 100644
--- a/weed-fs/src/pkg/storage/store.go
+++ b/weed-fs/src/pkg/storage/store.go
@@ -16,9 +16,9 @@ type Store struct {
 	Port         int
 	PublicServer string
 }
-type VolumeStat struct {
-	Id       uint64 "id"
-	CanWrite bool
+type VolumeInfo struct {
+	Id       uint64
+	Size int64
 }
 
 func NewStore(port int, publicServer, dirname string, chunkSize, capacity int) (s *Store) {
@@ -44,10 +44,10 @@ func NewStore(port int, publicServer, dirname string, chunkSize, capacity int) (
 }
 
 func (s *Store) Join(mserver string) {
-	stats := new([]*VolumeStat)
-	for k, _ := range s.volumes {
-		s := new(VolumeStat)
-		s.Id, s.CanWrite = k, true
+	stats := new([]*VolumeInfo)
+	for k, v := range s.volumes {
+		s := new(VolumeInfo)
+		s.Id, s.Size = k, v.Size()
 		*stats = append(*stats, s)
 	}
 	bytes, _ := json.Marshal(stats)
diff --git a/weed-fs/src/pkg/storage/volume.go b/weed-fs/src/pkg/storage/volume.go
index 3acd2d0e3..4b7f6ab3a 100644
--- a/weed-fs/src/pkg/storage/volume.go
+++ b/weed-fs/src/pkg/storage/volume.go
@@ -38,12 +38,12 @@ func NewVolume(dirname string, id uint64) (v *Volume) {
 
 	return
 }
-func (v *Volume) CanWrite(limit int64) bool {
+func (v *Volume) Size() int64 {
     stat, e:=v.dataFile.Stat()
     if e!=nil{
-       return stat.Size < limit
+       return stat.Size
     }
-    return false
+    return -1
 }
 func (v *Volume) Close() {
 	close(v.accessChannel)