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/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