var ok bool
	// Lookup in-memory drivercache
	d, ok = config.drivercache[tierName]
	if ok {
		return d, nil
	}

	// Initialize driver from tier config matching tierName
	t, ok := config.Tiers[tierName]
	if !ok {
		return nil, errTierNotFound
	}
	d, err = newWarmBackend(ctx, t, false)
	if err != nil {
		return nil, err
	}
	config.drivercache[tierName] = d
	return d, nil
}