levels.go
func (l *levelHandler) isCompactable(delSize int64) bool {
	return l.getTotalSize()-delSize >= l.maxTotalSize
}
func (l *levelHandler) isCompactable(delSize int64) bool {
	return l.getTotalSize()-delSize >= l.maxTotalSize
}
// Returns true if the non-zero level may be compacted.  delSize provides the size of the tables
// which are currently being compacted so that we treat them as already having started being
// compacted (because they have been, yet their size is already counted in getTotalSize).