diff --git a/archive.go b/archive.go
index ee8860c..18f8c32 100644
--- a/archive.go
+++ b/archive.go
@@ -45,7 +45,7 @@ func (m *MemoryStore) ToArchive(archiveRoot string, from, to int64) (int, error)
 
 	for i := 0; i < len(levels); i++ {
 		dir := path.Join(archiveRoot, path.Join(selectors[i]...))
-		err := levels[i].toArchive(dir, from, to)
+		err := levels[i].toArchive(dir, from, to, m)
 		if err != nil {
 			return i, err
 		}
@@ -54,7 +54,7 @@ func (m *MemoryStore) ToArchive(archiveRoot string, from, to int64) (int, error)
 	return len(levels), nil
 }
 
-func (l *level) toArchiveFile(from, to int64) (*ArchiveFile, error) {
+func (l *level) toArchiveFile(from, to int64, m *MemoryStore) (*ArchiveFile, error) {
 	l.lock.RLock()
 	defer l.lock.RUnlock()
 
@@ -64,7 +64,12 @@ func (l *level) toArchiveFile(from, to int64) (*ArchiveFile, error) {
 		Children: make(map[string]*ArchiveFile),
 	}
 
-	for metric, b := range l.metrics {
+	for metric, minfo := range m.metrics {
+		b := l.metrics[minfo.offset]
+		if b == nil {
+			continue
+		}
+
 		data := make([]Float, (to-from)/b.frequency+1)
 		data, start, end, err := b.read(from, to, data)
 		if err != nil {
@@ -83,7 +88,7 @@ func (l *level) toArchiveFile(from, to int64) (*ArchiveFile, error) {
 	}
 
 	for name, child := range l.children {
-		val, err := child.toArchiveFile(from, to)
+		val, err := child.toArchiveFile(from, to, m)
 		if err != nil {
 			return nil, err
 		}
@@ -94,8 +99,8 @@ func (l *level) toArchiveFile(from, to int64) (*ArchiveFile, error) {
 	return retval, nil
 }
 
-func (l *level) toArchive(dir string, from, to int64) error {
-	af, err := l.toArchiveFile(from, to)
+func (l *level) toArchive(dir string, from, to int64, m *MemoryStore) error {
+	af, err := l.toArchiveFile(from, to, m)
 	if err != nil {
 		return err
 	}
@@ -126,10 +131,10 @@ func (l *level) toArchive(dir string, from, to int64) error {
 // This function can only be called once and before the very first write or read.
 // Unlike ToArchive, this function is NOT thread-safe.
 func (m *MemoryStore) FromArchive(archiveRoot string, from int64) (int, error) {
-	return m.root.fromArchive(archiveRoot, from)
+	return m.root.fromArchive(archiveRoot, from, m)
 }
 
-func (l *level) loadFile(af *ArchiveFile) error {
+func (l *level) loadFile(af *ArchiveFile, m *MemoryStore) error {
 	for name, metric := range af.Metrics {
 		n := len(metric.Data)
 		b := &buffer{
@@ -140,9 +145,14 @@ func (l *level) loadFile(af *ArchiveFile) error {
 			next:      nil,
 		}
 
-		prev, ok := l.metrics[name]
+		minfo, ok := m.metrics[name]
 		if !ok {
-			l.metrics[name] = b
+			return errors.New("Unkown metric: " + name)
+		}
+
+		prev := l.metrics[minfo.offset]
+		if prev == nil {
+			l.metrics[minfo.offset] = b
 		} else {
 			if prev.start > b.start {
 				return errors.New("wooops")
@@ -151,20 +161,20 @@ func (l *level) loadFile(af *ArchiveFile) error {
 			b.prev = prev
 			prev.next = b
 		}
-		l.metrics[name] = b
+		l.metrics[minfo.offset] = b
 	}
 
 	for sel, childAf := range af.Children {
 		child, ok := l.children[sel]
 		if !ok {
 			child = &level{
-				metrics:  make(map[string]*buffer),
+				metrics:  make([]*buffer, len(m.metrics)),
 				children: make(map[string]*level),
 			}
 			l.children[sel] = child
 		}
 
-		err := child.loadFile(childAf)
+		err := child.loadFile(childAf, m)
 		if err != nil {
 			return err
 		}
@@ -173,7 +183,7 @@ func (l *level) loadFile(af *ArchiveFile) error {
 	return nil
 }
 
-func (l *level) fromArchive(dir string, from int64) (int, error) {
+func (l *level) fromArchive(dir string, from int64, m *MemoryStore) (int, error) {
 	direntries, err := os.ReadDir(dir)
 	if err != nil {
 		return 0, err
@@ -184,11 +194,11 @@ func (l *level) fromArchive(dir string, from int64) (int, error) {
 	for _, e := range direntries {
 		if e.IsDir() {
 			child := &level{
-				metrics:  make(map[string]*buffer),
+				metrics:  make([]*buffer, len(m.metrics)),
 				children: make(map[string]*level),
 			}
 
-			files, err := child.fromArchive(path.Join(dir, e.Name()), from)
+			files, err := child.fromArchive(path.Join(dir, e.Name()), from, m)
 			filesLoaded += files
 			if err != nil {
 				return filesLoaded, err
@@ -219,7 +229,7 @@ func (l *level) fromArchive(dir string, from int64) (int, error) {
 			return filesLoaded, err
 		}
 
-		err = l.loadFile(af)
+		err = l.loadFile(af, m)
 		if err != nil {
 			return filesLoaded, err
 		}
diff --git a/memstore.go b/memstore.go
index 80f2943..81966bf 100644
--- a/memstore.go
+++ b/memstore.go
@@ -153,53 +153,65 @@ func (b *buffer) free(t int64) (int, error) {
 // Can be both a leaf or a inner node. In this tree structue, inner nodes can
 // also hold data (in `metrics`).
 type level struct {
-	lock     sync.RWMutex       //
-	metrics  map[string]*buffer // Every level can store metrics.
-	children map[string]*level  // Lower levels.
+	lock     sync.RWMutex
+	metrics  []*buffer         // Every level can store metrics.
+	children map[string]*level // Lower levels.
 }
 
 // Find the correct level for the given selector, creating it if
 // it does not exist. Example selector in the context of the
 // ClusterCockpit could be: []string{ "emmy", "host123", "cpu", "0" }
 // This function would probably benefit a lot from `level.children` beeing a `sync.Map`?
-func (l *level) findLevelOrCreate(selector []string) *level {
+func (l *level) findLevelOrCreate(selector []string, nMetrics int) *level {
 	if len(selector) == 0 {
 		return l
 	}
 
 	// Allow concurrent reads:
 	l.lock.RLock()
-	child, ok := l.children[selector[0]]
-	l.lock.RUnlock()
-	if ok {
-		return child.findLevelOrCreate(selector[1:])
+	var child *level
+	var ok bool
+	if l.children == nil {
+		// Children map needs to be created...
+		l.lock.RUnlock()
+	} else {
+		child, ok := l.children[selector[0]]
+		l.lock.RUnlock()
+		if ok {
+			return child.findLevelOrCreate(selector[1:], nMetrics)
+		}
 	}
 
 	// The level does not exist, take write lock for unqiue access:
 	l.lock.Lock()
 	// While this thread waited for the write lock, another thread
 	// could have created the child node.
-	child, ok = l.children[selector[0]]
-	if ok {
-		l.lock.Unlock()
-		return child.findLevelOrCreate(selector[1:])
+	if l.children != nil {
+		child, ok = l.children[selector[0]]
+		if ok {
+			l.lock.Unlock()
+			return child.findLevelOrCreate(selector[1:], nMetrics)
+		}
+	} else {
+		l.children = make(map[string]*level)
 	}
 
 	child = &level{
-		metrics:  make(map[string]*buffer),
-		children: make(map[string]*level),
+		metrics:  make([]*buffer, nMetrics),
+		children: nil,
 	}
+
 	l.children[selector[0]] = child
 	l.lock.Unlock()
-	return child.findLevelOrCreate(selector[1:])
+	return child.findLevelOrCreate(selector[1:], nMetrics)
 }
 
 // This function assmumes that `l.lock` is LOCKED!
 // Read `buffer.read` for context.
 // If this level does not have data for the requested metric, the data
 // is aggregated timestep-wise from all the children (recursively).
-func (l *level) read(metric string, from, to int64, data []Float) ([]Float, int, int64, int64, error) {
-	if b, ok := l.metrics[metric]; ok {
+func (l *level) read(offset int, from, to int64, data []Float) ([]Float, int, int64, int64, error) {
+	if b := l.metrics[offset]; b != nil {
 		// Whoo, this is the "native" level of this metric:
 		data, from, to, err := b.read(from, to, data)
 		return data, 1, from, to, err
@@ -212,7 +224,7 @@ func (l *level) read(metric string, from, to int64, data []Float) ([]Float, int,
 	n := 0
 	for _, child := range l.children {
 		child.lock.RLock()
-		cdata, cn, cfrom, cto, err := child.read(metric, from, to, data)
+		cdata, cn, cfrom, cto, err := child.read(offset, from, to, data)
 		child.lock.RUnlock()
 
 		if err == ErrNoData {
@@ -255,6 +267,10 @@ func (l *level) free(t int64) (int, error) {
 
 	n := 0
 	for _, b := range l.metrics {
+		if b == nil {
+			continue
+		}
+
 		m, err := b.free(t)
 		n += m
 		if err != nil {
@@ -273,40 +289,81 @@ func (l *level) free(t int64) (int, error) {
 	return n, nil
 }
 
+type AggregationStrategy int
+
+const (
+	NoAggregation AggregationStrategy = iota
+	SumAggregation
+	AvgAggregation
+)
+
 type MemoryStore struct {
 	root    level // root of the tree structure
-	metrics map[string]MetricConfig
+	metrics map[string]struct {
+		offset      int
+		aggregation AggregationStrategy
+		frequency   int64
+	}
 }
 
 func NewMemoryStore(metrics map[string]MetricConfig) *MemoryStore {
+	ms := make(map[string]struct {
+		offset      int
+		aggregation AggregationStrategy
+		frequency   int64
+	})
+
+	offset := 0
+	for key, config := range metrics {
+		aggregation := NoAggregation
+		if config.Aggregation == "sum" {
+			aggregation = SumAggregation
+		} else if config.Aggregation == "avg" {
+			aggregation = AvgAggregation
+		} else if config.Aggregation != "" {
+			panic("invalid aggregation strategy: " + config.Aggregation)
+		}
+
+		ms[key] = struct {
+			offset      int
+			aggregation AggregationStrategy
+			frequency   int64
+		}{
+			offset:      offset,
+			aggregation: aggregation,
+			frequency:   config.Frequency,
+		}
+
+		offset += 1
+	}
+
 	return &MemoryStore{
 		root: level{
-			metrics:  make(map[string]*buffer),
+			metrics:  make([]*buffer, len(metrics)),
 			children: make(map[string]*level),
 		},
-		metrics: metrics,
+		metrics: ms,
 	}
 }
 
 // Write all values in `metrics` to the level specified by `selector` for time `ts`.
 // Look at `findLevelOrCreate` for how selectors work.
 func (m *MemoryStore) Write(selector []string, ts int64, metrics []Metric) error {
-	l := m.root.findLevelOrCreate(selector)
+	l := m.root.findLevelOrCreate(selector, len(m.metrics))
 	l.lock.Lock()
 	defer l.lock.Unlock()
 
 	for _, metric := range metrics {
-		b, ok := l.metrics[metric.Name]
+		minfo, ok := m.metrics[metric.Name]
 		if !ok {
-			minfo, ok := m.metrics[metric.Name]
-			if !ok {
-				// return errors.New("unkown metric: " + metric.Name)
-				continue
-			}
+			continue
+		}
 
+		b := l.metrics[minfo.offset]
+		if b == nil {
 			// First write to this metric and level
-			b = newBuffer(ts, minfo.Frequency)
-			l.metrics[metric.Name] = b
+			b = newBuffer(ts, minfo.frequency)
+			l.metrics[minfo.offset] = b
 		}
 
 		nb, err := b.write(ts, metric.Value)
@@ -316,7 +373,7 @@ func (m *MemoryStore) Write(selector []string, ts int64, metrics []Metric) error
 
 		// Last write created a new buffer...
 		if b != nb {
-			l.metrics[metric.Name] = nb
+			l.metrics[minfo.offset] = nb
 		}
 	}
 	return nil
@@ -326,7 +383,7 @@ func (m *MemoryStore) Write(selector []string, ts int64, metrics []Metric) error
 // If the level does not hold the metric itself, the data will be aggregated recursively from the children.
 // See `level.read` for more information.
 func (m *MemoryStore) Read(selector []string, metric string, from, to int64) ([]Float, int64, int64, error) {
-	l := m.root.findLevelOrCreate(selector)
+	l := m.root.findLevelOrCreate(selector, len(m.metrics))
 	l.lock.RLock()
 	defer l.lock.RUnlock()
 
@@ -339,20 +396,20 @@ func (m *MemoryStore) Read(selector []string, metric string, from, to int64) ([]
 		return nil, 0, 0, errors.New("unkown metric: " + metric)
 	}
 
-	data := make([]Float, (to-from)/minfo.Frequency)
-	data, n, from, to, err := l.read(metric, from, to, data)
+	data := make([]Float, (to-from)/minfo.frequency+1)
+	data, n, from, to, err := l.read(minfo.offset, from, to, data)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 
 	if n > 1 {
-		if minfo.Aggregation == "avg" {
+		if minfo.aggregation == AvgAggregation {
 			normalize := 1. / Float(n)
 			for i := 0; i < len(data); i++ {
 				data[i] *= normalize
 			}
-		} else if minfo.Aggregation != "sum" {
-			return nil, 0, 0, errors.New("invalid aggregation strategy: " + minfo.Aggregation)
+		} else if minfo.aggregation != SumAggregation {
+			return nil, 0, 0, errors.New("invalid aggregation")
 		}
 	}
 
@@ -362,5 +419,5 @@ func (m *MemoryStore) Read(selector []string, metric string, from, to int64) ([]
 // Release all buffers for the selected level and all its children that contain only
 // values older than `t`.
 func (m *MemoryStore) Free(selector []string, t int64) (int, error) {
-	return m.root.findLevelOrCreate(selector).free(t)
+	return m.root.findLevelOrCreate(selector, len(m.metrics)).free(t)
 }
diff --git a/stats.go b/stats.go
index e47f74d..53e4e9a 100644
--- a/stats.go
+++ b/stats.go
@@ -61,8 +61,8 @@ func (b *buffer) stats(from, to int64) (Stats, int64, int64, error) {
 // This function assmumes that `l.lock` is LOCKED!
 // It basically works just like level.read but calculates min/max/avg for that data level.read would return.
 // TODO: Make this DRY?
-func (l *level) stats(metric string, from, to int64, aggregation string) (Stats, int64, int64, error) {
-	if b, ok := l.metrics[metric]; ok {
+func (l *level) stats(offset int, from, to int64, aggreg AggregationStrategy) (Stats, int64, int64, error) {
+	if b := l.metrics[offset]; b != nil {
 		return b.stats(from, to)
 	}
 
@@ -75,7 +75,7 @@ func (l *level) stats(metric string, from, to int64, aggregation string) (Stats,
 	avg, min, max := Float(0), Float(math.MaxFloat32), Float(-math.MaxFloat32)
 	for _, child := range l.children {
 		child.lock.RLock()
-		stats, cfrom, cto, err := child.stats(metric, from, to, aggregation)
+		stats, cfrom, cto, err := child.stats(offset, from, to, aggreg)
 		child.lock.RUnlock()
 
 		if err == ErrNoData {
@@ -104,10 +104,10 @@ func (l *level) stats(metric string, from, to int64, aggregation string) (Stats,
 		return Stats{}, 0, 0, ErrNoData
 	}
 
-	if aggregation == "avg" {
+	if aggreg == AvgAggregation {
 		avg /= Float(n)
-	} else if aggregation != "sum" {
-		return Stats{}, 0, 0, errors.New("invalid aggregation strategy: " + aggregation)
+	} else if aggreg != SumAggregation {
+		return Stats{}, 0, 0, errors.New("invalid aggregation")
 	}
 
 	return Stats{
@@ -119,7 +119,7 @@ func (l *level) stats(metric string, from, to int64, aggregation string) (Stats,
 }
 
 func (m *MemoryStore) Stats(selector []string, metric string, from, to int64) (*Stats, int64, int64, error) {
-	l := m.root.findLevelOrCreate(selector)
+	l := m.root.findLevelOrCreate(selector, len(m.metrics))
 	l.lock.RLock()
 	defer l.lock.RUnlock()
 
@@ -132,6 +132,6 @@ func (m *MemoryStore) Stats(selector []string, metric string, from, to int64) (*
 		return nil, 0, 0, errors.New("unkown metric: " + metric)
 	}
 
-	stats, from, to, err := l.stats(metric, from, to, minfo.Aggregation)
+	stats, from, to, err := l.stats(minfo.offset, from, to, minfo.aggregation)
 	return &stats, from, to, err
 }