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 66c3254..584842d 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
@@ -190,7 +190,7 @@ public class PersistentMap<K, V> implements AutoCloseable {
 
     /**
      * 
-     * @param path            file relative to {@code storageBasePath}
+     * @param path            file for the index, must be child of storageBasePath
      * @param storageBasePath base path
      * @param keyEncoder      encoder for keys
      * @param valueEncoder    encoder for values
diff --git a/pdb-api/build.gradle b/pdb-api/build.gradle
index f7960ff..c891eb5 100644
--- a/pdb-api/build.gradle
+++ b/pdb-api/build.gradle
@@ -1,5 +1,6 @@
 
 dependencies {
 	implementation project(':pdb-utils')
+	implementation project(':block-storage')
   implementation lib_primitive_collections
 }
\ No newline at end of file
diff --git a/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java b/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
index 61a8cf5..007a514 100644
--- a/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
+++ b/pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java
@@ -2,11 +2,8 @@ package org.lucares.pdb.api;
 
 import java.io.BufferedReader;
 import java.io.FileInputStream;
-import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStreamReader;
-import java.io.OutputStreamWriter;
-import java.io.Writer;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -19,6 +16,8 @@ import java.util.Set;
 import java.util.function.Function;
 import java.util.regex.Pattern;
 
+import org.lucares.pdb.map.PersistentMap;
+
 /**
  * A very simple {@link Set}-like or {@link Map}-like data structure that stores
  * unique&sup1; pairs of Strings and integers persistently.
@@ -33,8 +32,6 @@ import java.util.regex.Pattern;
 public class UniqueStringIntegerPairs {
     private static final String SEPARATOR = "\t";
 
-    private static final boolean APPEND = true;
-
     private static final class ByteArray implements Comparable<ByteArray> {
         private final byte[] array;
         private final int start;
@@ -97,37 +94,50 @@ public class UniqueStringIntegerPairs {
      */
     private final List<String> intToString = new ArrayList<>();
 
-    private final Path file;
+    final PersistentMap<String, Long> persistentMap;
 
     public UniqueStringIntegerPairs() {
         this(null);
     }
 
     public UniqueStringIntegerPairs(final Path storageBasePath) {
-        this.file = keyCompressionFile(storageBasePath);
-        if (file != null) {
-            init(file);
+
+        if (storageBasePath != null) {
+            persistentMap = new PersistentMap<>(storageBasePath.resolve("keys.bs"), storageBasePath,
+                    PersistentMap.STRING_CODER, PersistentMap.LONG_CODER);
+
+            final Path oldKeysCsvFile = keyCompressionFile(storageBasePath);
+            if (persistentMap.isEmpty() && Files.exists(oldKeysCsvFile)) {
+                upgradeFromCsvFile(oldKeysCsvFile);
+            } else {
+                init();
+            }
+        } else {
+            // some unit tests disable the persistence and use this class memory only
+            persistentMap = null;
         }
     }
 
+    private void init() {
+        persistentMap.forAll((string, integer) -> {
+            intToStringPut(integer.intValue(), string);
+            stringToInt.put(string, integer.intValue());
+            bytesToInt.put(new ByteArray(string), integer.intValue());
+        });
+    }
+
     private Path keyCompressionFile(final Path dataDirectory) {
         return dataDirectory.resolve("keys.csv");
     }
 
-    private void init(final Path file) throws RuntimeIOException {
+    private void upgradeFromCsvFile(final Path file) throws RuntimeIOException {
 
         try {
-            Files.createDirectories(file.getParent());
-            if (!Files.exists(file)) {
-                Files.createFile(file);
-            }
-
             try (final BufferedReader reader = new BufferedReader(
                     new InputStreamReader(new FileInputStream(file.toFile()), StandardCharsets.UTF_8))) {
                 String line;
                 while ((line = reader.readLine()) != null) {
 
-                    // TODO use more efficient code to read the CSV -> improves startup time
                     final String[] tokens = line.split(Pattern.quote(SEPARATOR));
 
                     if (tokens.length == 2) {
@@ -136,6 +146,7 @@ public class UniqueStringIntegerPairs {
                         intToStringPut(integer, string);
                         stringToInt.put(string, integer);
                         bytesToInt.put(new ByteArray(string), integer);
+                        persistentMap.putValue(string, (long) integer);
                     }
                 }
             }
@@ -159,15 +170,8 @@ public class UniqueStringIntegerPairs {
         if (stringToInt.containsKey(string) || (intToString.size() > integer && intToString.get(integer) != null)) {
             throw new IllegalArgumentException("Unique key constraint violation for (" + string + ", " + integer + ")");
         }
-        if (file != null) {
-            try (final Writer writer = new OutputStreamWriter(new FileOutputStream(file.toFile(), APPEND),
-                    StandardCharsets.UTF_8)) {
-
-                writer.write(string + SEPARATOR + integer + "\n");
-
-            } catch (final IOException e) {
-                throw new RuntimeIOException(e);
-            }
+        if (persistentMap != null) {
+            persistentMap.putValue(string, (long) integer);
         }
 
         intToStringPut(integer, string);