directory structure change to work with glide

glide has its own requirements. My previous workaround caused me some
code checkin errors. Need to fix this.

weed/storage/volume_super_block.go

@@ -0,0 +1,81 @@
+package storage
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/chrislusf/seaweedfs/weed/glog"
+	"github.com/chrislusf/seaweedfs/weed/util"
+)
+
+const (
+	SuperBlockSize = 8
+)
+
+/*
+* Super block currently has 8 bytes allocated for each volume.
+* Byte 0: version, 1 or 2
+* Byte 1: Replica Placement strategy, 000, 001, 002, 010, etc
+* Byte 2 and byte 3: Time to live. See TTL for definition
+* Byte 4 and byte 5: The number of times the volume has been compacted.
+* Rest bytes: Reserved
+ */
+type SuperBlock struct {
+	version          Version
+	ReplicaPlacement *ReplicaPlacement
+	Ttl              *TTL
+	CompactRevision  uint16
+}
+
+func (s *SuperBlock) Version() Version {
+	return s.version
+}
+func (s *SuperBlock) Bytes() []byte {
+	header := make([]byte, SuperBlockSize)
+	header[0] = byte(s.version)
+	header[1] = s.ReplicaPlacement.Byte()
+	s.Ttl.ToBytes(header[2:4])
+	util.Uint16toBytes(header[4:6], s.CompactRevision)
+	return header
+}
+
+func (v *Volume) maybeWriteSuperBlock() error {
+	stat, e := v.dataFile.Stat()
+	if e != nil {
+		glog.V(0).Infof("failed to stat datafile %s: %v", v.dataFile, e)
+		return e
+	}
+	if stat.Size() == 0 {
+		v.SuperBlock.version = CurrentVersion
+		_, e = v.dataFile.Write(v.SuperBlock.Bytes())
+		if e != nil && os.IsPermission(e) {
+			//read-only, but zero length - recreate it!
+			if v.dataFile, e = os.Create(v.dataFile.Name()); e == nil {
+				if _, e = v.dataFile.Write(v.SuperBlock.Bytes()); e == nil {
+					v.readOnly = false
+				}
+			}
+		}
+	}
+	return e
+}
+func (v *Volume) readSuperBlock() (err error) {
+	if _, err = v.dataFile.Seek(0, 0); err != nil {
+		return fmt.Errorf("cannot seek to the beginning of %s: %v", v.dataFile.Name(), err)
+	}
+	header := make([]byte, SuperBlockSize)
+	if _, e := v.dataFile.Read(header); e != nil {
+		return fmt.Errorf("cannot read volume %d super block: %v", v.Id, e)
+	}
+	v.SuperBlock, err = ParseSuperBlock(header)
+	return err
+}
+func ParseSuperBlock(header []byte) (superBlock SuperBlock, err error) {
+	superBlock.version = Version(header[0])
+	if superBlock.ReplicaPlacement, err = NewReplicaPlacementFromByte(header[1]); err != nil {
+		err = fmt.Errorf("cannot read replica type: %s", err.Error())
+	}
+	superBlock.Ttl = LoadTTLFromBytes(header[2:4])
+	superBlock.CompactRevision = util.BytesToUint16(header[4:6])
+	return
+}