diff --git a/weed-fs/src/pkg/directory/volume_mapping.go b/weed-fs/src/pkg/directory/volume_mapping.go
index 828975b97..b4862d70f 100644
--- a/weed-fs/src/pkg/directory/volume_mapping.go
+++ b/weed-fs/src/pkg/directory/volume_mapping.go
@@ -20,8 +20,8 @@ type MachineInfo struct {
 	PublicUrl string
 }
 type Machine struct {
-	Server   MachineInfo
-	Volumes  []storage.VolumeInfo
+	Server  MachineInfo
+	Volumes []storage.VolumeInfo
 }
 
 type Mapper struct {
@@ -30,12 +30,12 @@ type Mapper struct {
 
 	lock          sync.Mutex
 	Machines      []*Machine
-	vid2machineId map[uint32]int
-	Writers       []int // transient array of Writers volume id
+	vid2machineId map[uint32]int //machineId is +1 of the index of []*Machine, to detect not found entries
+	Writers       []uint32 // transient array of Writers volume id
 
 	FileIdSequence uint64
 	fileIdCounter  uint64
-	
+
 	volumeSizeLimit uint32
 }
 
@@ -47,7 +47,7 @@ func NewMapper(dirname string, filename string, volumeSizeLimit uint32) (m *Mapp
 	m = &Mapper{dir: dirname, fileName: filename}
 	m.vid2machineId = make(map[uint32]int)
 	m.volumeSizeLimit = volumeSizeLimit
-	m.Writers = *new([]int)
+	m.Writers = *new([]uint32)
 	m.Machines = *new([]*Machine)
 
 	seqFile, se := os.OpenFile(path.Join(m.dir, m.fileName+".seq"), os.O_RDONLY, 0644)
@@ -65,14 +65,18 @@ func NewMapper(dirname string, filename string, volumeSizeLimit uint32) (m *Mapp
 	return
 }
 func (m *Mapper) PickForWrite() (string, MachineInfo, os.Error) {
-    len_writers := len(m.Writers)
-    if len_writers<=0 {
-        log.Println("No more writable volumes!")
-        return "",m.Machines[rand.Intn(len(m.Machines))].Server, os.NewError("No more writable volumes!")
-    }
-	machine := m.Machines[m.Writers[rand.Intn(len_writers)]]
-	vid := machine.Volumes[rand.Intn(len(machine.Volumes))].Id
-	return NewFileId(vid, m.NextFileId(), rand.Uint32()).String(), machine.Server,nil
+	len_writers := len(m.Writers)
+	if len_writers <= 0 {
+		log.Println("No more writable volumes!")
+		return "", m.Machines[rand.Intn(len(m.Machines))].Server, os.NewError("No more writable volumes!")
+	}
+	vid := m.Writers[rand.Intn(len_writers)]
+	machine_id := m.vid2machineId[vid]
+	if machine_id > 0 {
+		machine := m.Machines[machine_id-1]
+		return NewFileId(vid, m.NextFileId(), rand.Uint32()).String(), machine.Server, nil
+	}
+	return "", m.Machines[rand.Intn(len(m.Machines))].Server, os.NewError("Strangely vid " + strconv.Uitoa64(uint64(vid)) + " is on no machine!")
 }
 func (m *Mapper) NextFileId() uint64 {
 	if m.fileIdCounter <= 0 {
@@ -84,13 +88,13 @@ func (m *Mapper) NextFileId() uint64 {
 	return m.FileIdSequence - m.fileIdCounter
 }
 func (m *Mapper) Get(vid uint32) (*Machine, os.Error) {
-    machineId := m.vid2machineId[vid]
-    if machineId <=0{
-        return nil, os.NewError("invalid volume id " + strconv.Uitob64(uint64(vid),10))
-    }
-	return m.Machines[machineId-1],nil
+	machineId := m.vid2machineId[vid]
+	if machineId <= 0 {
+		return nil, os.NewError("invalid volume id " + strconv.Uitob64(uint64(vid), 10))
+	}
+	return m.Machines[machineId-1], nil
 }
-func (m *Mapper) Add(machine Machine){
+func (m *Mapper) Add(machine Machine) {
 	//check existing machine, linearly
 	m.lock.Lock()
 	foundExistingMachineId := -1
@@ -104,22 +108,22 @@ func (m *Mapper) Add(machine Machine){
 	if machineId < 0 {
 		machineId = len(m.Machines)
 		m.Machines = append(m.Machines, &machine)
-	}else{
-	    m.Machines[machineId] = &machine
+	} else {
+		m.Machines[machineId] = &machine
 	}
 	m.lock.Unlock()
 
 	//add to vid2machineId map, and Writers array
 	for _, v := range machine.Volumes {
 		//log.Println("Setting volume", v.Id, "to", machine.Server.Url)
-		m.vid2machineId[v.Id] = machineId+1 //use base 1 indexed, to detect not found cases
+		m.vid2machineId[v.Id] = machineId + 1 //use base 1 indexed, to detect not found cases
 	}
 	//setting Writers, copy-on-write because of possible updating
-	var writers []int
-	for machine_index, machine_entry := range m.Machines {
+	var writers []uint32
+	for _, machine_entry := range m.Machines {
 		for _, v := range machine_entry.Volumes {
 			if v.Size < int64(m.volumeSizeLimit) {
-				writers = append(writers, machine_index)
+				writers = append(writers, v.Id)
 			}
 		}
 	}