adding ttl field to volume super block

go/storage/volume_super_block.go

@@ -2,6 +2,7 @@ package storage
 
 import (
 	"code.google.com/p/weed-fs/go/glog"
+	"code.google.com/p/weed-fs/go/util"
 	"fmt"
 	"os"
 )
@@ -10,9 +11,16 @@ 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 in minutes
+ */
 type SuperBlock struct {
 	version          Version
 	ReplicaPlacement *ReplicaPlacement
+	Ttl              uint16
 }
 
 func (s *SuperBlock) Version() Version {
@@ -22,6 +30,7 @@ func (s *SuperBlock) Bytes() []byte {
 	header := make([]byte, SuperBlockSize)
 	header[0] = byte(s.version)
 	header[1] = s.ReplicaPlacement.Byte()
+	util.Uint16toBytes(header[2:4], s.Ttl)
 	return header
 }
 
@@ -61,5 +70,6 @@ func ParseSuperBlock(header []byte) (superBlock SuperBlock, err error) {
 	if superBlock.ReplicaPlacement, err = NewReplicaPlacementFromByte(header[1]); err != nil {
 		err = fmt.Errorf("cannot read replica type: %s", err.Error())
 	}
+	superBlock.Ttl = util.BytesToUint16(header[2:4])
 	return
 }