// the 'writeQuorum' this function returns "" if quorum cannot
// be achieved and disks have too many inconsistent versions.
func reduceCommonVersions(diskVersions [][]byte, writeQuorum int) (versions []byte) {
	diskVersionsCount := make(map[uint64]int)
	for _, versions := range diskVersions {
		if len(versions) > 0 {
			diskVersionsCount[binary.BigEndian.Uint64(versions)]++
		}
	}

	var commonVersions uint64
	maxCnt := 0