allocate volume by disk type

weed/storage/volume.go

@@ -171,6 +171,10 @@ func (v *Volume) IndexFileSize() uint64 {
 	return v.nm.IndexFileSize()
 }
 
+func (v *Volume) DiskType() DiskType {
+	return v.location.DiskType
+}
+
 // Close cleanly shuts down this volume
 func (v *Volume) Close() {
 	v.dataFileAccessLock.Lock()
@@ -262,7 +266,7 @@ func (v *Volume) ToVolumeInformationMessage() (types.NeedleId, *master_pb.Volume
 		Ttl:              v.Ttl.ToUint32(),
 		CompactRevision:  uint32(v.SuperBlock.CompactionRevision),
 		ModifiedAtSecond: modTime.Unix(),
-		DiskType:       string(v.location.DiskType),
+		DiskType:         string(v.location.DiskType),
 	}
 
 	volumeInfo.RemoteStorageName, volumeInfo.RemoteStorageKey = v.RemoteStorageNameKey()