func (pc *Periodic) getCompactInterval() time.Duration {
	itv := pc.period
	if itv > time.Hour {
		itv = time.Hour
	}
	return itv
}
func (pc *Periodic) getCompactInterval() time.Duration {
	itv := pc.period
	if itv > time.Hour {
		itv = time.Hour
	}
	return itv
}
// if given compaction period x is <1-hour, compact every x duration.
// (e.g. --auto-compaction-mode 'periodic' --auto-compaction-retention='10m', then compact every 10-minute)
// if given compaction period x is >1-hour, compact every hour.
// (e.g. --auto-compaction-mode 'periodic' --auto-compaction-retention='2h', then compact every 1-hour)