diff --git a/block-storage/src/main/java/org/lucares/pdb/map/ByteArrayKey.java b/block-storage/src/main/java/org/lucares/pdb/map/ByteArrayKey.java
index c15b220..63b3e3b 100644
--- a/block-storage/src/main/java/org/lucares/pdb/map/ByteArrayKey.java
+++ b/block-storage/src/main/java/org/lucares/pdb/map/ByteArrayKey.java
@@ -48,6 +48,11 @@ public final class ByteArrayKey implements Comparable<ByteArrayKey> {
 	public static boolean equal(final byte[] key, final byte[] otherKey) {
 		return compare(key, otherKey) == 0;
 	}
+	
+	@Override
+	public String toString() {
+		return Arrays.toString(bytes);
+	}
 
 	@Override
 	public int hashCode() {
diff --git a/block-storage/src/main/java/org/lucares/pdb/map/NodeEntry.java b/block-storage/src/main/java/org/lucares/pdb/map/NodeEntry.java
index 4a0b366..c32df7a 100644
--- a/block-storage/src/main/java/org/lucares/pdb/map/NodeEntry.java
+++ b/block-storage/src/main/java/org/lucares/pdb/map/NodeEntry.java
@@ -81,15 +81,15 @@ class NodeEntry {
 				+ valueAsString + "]";
 	}
 
-	public String toString(final Function<byte[], String> keyDecoder, final Function<byte[], String> valueDecoder) {
+	public <K,V> String toString(final Function<byte[], K> keyDecoder, final Function<byte[], V> valueDecoder) {
 		final String valueAsString = isInnerNode() ? String.valueOf(VariableByteEncoder.decodeFirstValue(value))
-				: valueDecoder.apply(value);
+				: String.valueOf(valueDecoder.apply(value));
 
 		final String keyAsString;
 		if (Arrays.equals(key, PersistentMap.MAX_KEY)) {
 			keyAsString = "<<<MAX_KEY>>>";
 		} else {
-			keyAsString = keyDecoder.apply(key);
+			keyAsString = String.valueOf(keyDecoder.apply(key));
 		}
 
 		return "NodeEntry [type=" + type + ", key=" + keyAsString + ", value=" + valueAsString + "]";
diff --git a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
index c5efa6d..aa12ea8 100644
--- a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
+++ b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMap.java
@@ -7,6 +7,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.function.Function;
 import java.util.Objects;
 import java.util.Stack;
 import java.util.UUID;
@@ -39,6 +40,14 @@ public class PersistentMap<K, V> implements AutoCloseable {
 		public byte[] encode(O object);
 
 		public O decode(byte[] bytes);
+		
+		public default Function<byte[], O> asDecoder() {
+			return bytes -> this.decode(bytes);
+		}
+		
+		public default Function<O,byte[]> asEncoder() {
+			return plain -> this.encode(plain);
+		}
 	}
 
 	private static final class StringCoder implements EncoderDecoder<String> {
@@ -160,7 +169,7 @@ public class PersistentMap<K, V> implements AutoCloseable {
 			writeNodeOffsetOfRootNode(blockOffset);
 
 			// 5. insert a dummy entry with a 'maximum' key
-			putValue(MAX_KEY, new byte[] { 0 });
+			putValue(MAX_KEY, new byte[] { });
 		}
 	}
 
@@ -357,7 +366,9 @@ public class PersistentMap<K, V> implements AutoCloseable {
 	}
 
 	private void writeNode(final PersistentMapDiskNode node) {
-		LOGGER.trace("writing node {}", node);
+		if (LOGGER.isTraceEnabled()) {
+			LOGGER.trace("writing node {}", node.toString(keyEncoder.asDecoder(), valueEncoder.asDecoder()));
+		}
 		final long nodeOffest = node.getNodeOffset();
 		// final DiskBlock diskBlock = diskStore.getDiskBlock(nodeOffest, BLOCK_SIZE);
 		DiskBlock diskBlock = node.getDiskBlock();
diff --git a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMapDiskNode.java b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMapDiskNode.java
index 5f3edb0..2bb8e96 100644
--- a/block-storage/src/main/java/org/lucares/pdb/map/PersistentMapDiskNode.java
+++ b/block-storage/src/main/java/org/lucares/pdb/map/PersistentMapDiskNode.java
@@ -8,6 +8,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.TreeMap;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 import org.lucares.collections.LongList;
@@ -200,6 +201,20 @@ public class PersistentMapDiskNode {
 		return "@" + nodeOffset + ": "
 				+ String.join("\n", entries.values().stream().map(NodeEntry::toString).collect(Collectors.toList()));
 	}
+	
+	public <K,V> String toString(Function<byte[], K> keyDecoder, Function<byte[], V> valueDecoder) {
+		StringBuilder result = new StringBuilder();
+		result.append("@");
+		result.append(nodeOffset);
+		result.append(": ");
+		for (NodeEntry e : entries.values()) {
+			String s = e.toString(keyDecoder, valueDecoder);
+			result.append("\n");
+			result.append(s);
+		}
+		
+		return result.toString();
+	}
 
 	public NodeEntry getTopNodeEntry() {
 		return entries.lastEntry().getValue();
diff --git a/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java b/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
index 2e8e8aa..53f7dbb 100644
--- a/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
+++ b/block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java
@@ -179,6 +179,66 @@ public class PersistentMapTest {
 
 		}
 	}
+	@Test(invocationCount = 1)
+	public void testManyEmptyValues() throws Exception {
+		final Path file = dataDirectory.resolve("map.db");
+		final var insertedValues = new HashMap<Long, Empty>();
+		
+		final SecureRandom rnd = new SecureRandom();
+		rnd.setSeed(1);
+		
+		try (final PersistentMap<Long, Empty> map = new PersistentMap<>(file, PersistentMap.LONG_CODER,
+				PersistentMap.EMPTY_ENCODER)) {
+			
+			for (int i = 0; i < 1500; i++) {
+				// System.out.println("\n\ninserting: " + i);
+				
+				final Long key = (long) (rnd.nextGaussian() * Integer.MAX_VALUE);
+				final Empty value = Empty.INSTANCE;
+				Assert.assertNull(map.getValue(key));
+				
+				Assert.assertNull(map.putValue(key, value));
+				
+				insertedValues.put(key, value);
+				
+				// map.print();
+				
+				final boolean failEarly = false;
+				if (failEarly) {
+					for (final var entry : insertedValues.entrySet()) {
+						final Empty actualValue = map.getValue(entry.getKey());
+						
+						if (!Objects.equals(actualValue, entry.getValue())) {
+							map.print();
+						}
+						
+						Assert.assertEquals(actualValue, entry.getValue(),
+								"value for key " + entry.getKey() + " in the " + i + "th iteration");
+					}
+				}
+			}
+		}
+		
+		try (final PersistentMap<Long, Empty> map = new PersistentMap<>(file, PersistentMap.LONG_CODER,
+				PersistentMap.EMPTY_ENCODER)) {
+			map.print();
+			final AtomicInteger counter = new AtomicInteger();
+			map.visitNodeEntriesPreOrder(
+					(node, parentNode, nodeEntry, depth) -> counter.addAndGet(nodeEntry.isInnerNode() ? 1 : 0));
+			
+			Assert.assertEquals(counter.get(), 4,
+					"number of nodes should be small. Any number larger than 4 indicates, "
+							+ "that new inner nodes are created even though the existing inner "
+							+ "nodes could hold the values");
+			
+			for (final var entry : insertedValues.entrySet()) {
+				final Empty actualValue = map.getValue(entry.getKey());
+				Assert.assertEquals(actualValue, entry.getValue(),
+						"value for key " + entry.getKey() + " after all iterations");
+			}
+			
+		}
+	}
 
 	@Test(invocationCount = 1)
 	public void testEasyValues() throws Exception {
diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/PdbFile.java b/data-store/src/main/java/org/lucares/pdb/datastore/PdbFile.java
index c902f18..364b41c 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/PdbFile.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/PdbFile.java
@@ -66,7 +66,7 @@ public class PdbFile {
 
 	@Override
 	public String toString() {
-		return "PdbFile [tags=" + tags + ", rootBlockNumber=" + rootBlockNumber + "]";
+		return "PdbFile [tags=" + tags + ", rootBlockNumber=" + rootBlockNumber + ", partitionId="+partitionId+"]";
 	}
 
 	@Override
diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/internal/PdbWriter.java b/data-store/src/main/java/org/lucares/pdb/datastore/internal/PdbWriter.java
index 9ae5f47..ecb1a59 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/internal/PdbWriter.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/internal/PdbWriter.java
@@ -22,13 +22,13 @@ class PdbWriter implements AutoCloseable, Flushable {
 	private final PdbFile pdbFile;
 	private long lastEpochMilli;
 
-	private final TimeSeriesFile bsFile;
+	private final TimeSeriesFile timeSeriesFile;
 
 	public PdbWriter(final PdbFile pdbFile, final DiskStorage diskStorage) {
 		this.pdbFile = pdbFile;
 
-		bsFile = TimeSeriesFile.existingFile(pdbFile.getRootBlockNumber(), diskStorage);
-		final Optional<Long> optionalLastValue = bsFile.getLastValue(); // TODO is this last value correct?
+		timeSeriesFile = TimeSeriesFile.existingFile(pdbFile.getRootBlockNumber(), diskStorage);
+		final Optional<Long> optionalLastValue = timeSeriesFile.getLastValue(); // TODO is this last value correct?
 
 		lastEpochMilli = optionalLastValue.orElse(0L);
 	}
@@ -43,7 +43,7 @@ class PdbWriter implements AutoCloseable, Flushable {
 
 	public void write(final long epochMilli, final long value) throws WriteException, InvalidValueException {
 		try {
-			bsFile.appendTimeValue(epochMilli, value);
+			timeSeriesFile.appendTimeValue(epochMilli, value);
 
 			lastEpochMilli = epochMilli;
 		} catch (final RuntimeException e) {
@@ -55,12 +55,12 @@ class PdbWriter implements AutoCloseable, Flushable {
 	public void close() {
 
 		LOGGER.debug("close PdbWriter {}", pdbFile);
-		bsFile.close();
+		timeSeriesFile.close();
 	}
 
 	@Override
 	public void flush() {
-		bsFile.flush();
+		timeSeriesFile.flush();
 	}
 
 	public static void writeEntry(final PdbFile pdbFile, final DiskStorage diskStorage, final Entry... entries) {