diff --git a/data-store/src/main/java/org/lucares/pdb/datastore/internal/FolderStorage.java b/data-store/src/main/java/org/lucares/pdb/datastore/internal/FolderStorage.java
index c4abc1f..d19de2c 100644
--- a/data-store/src/main/java/org/lucares/pdb/datastore/internal/FolderStorage.java
+++ b/data-store/src/main/java/org/lucares/pdb/datastore/internal/FolderStorage.java
@@ -29,6 +29,7 @@ public class FolderStorage implements AutoCloseable {
 			.getLogger("org.lucares.metrics.folderStorage.createListingFile");
 	private final static Logger METRICS_GET_PATH_BY_OFFSET = LoggerFactory
 			.getLogger("org.lucares.metrics.folderStorage.getPathByOffset");
+	private final static Logger METRICS_INSERT = LoggerFactory.getLogger("org.lucares.metrics.folderStorage.insert");
 
 	private final Path storageBaseDirectory;
 
@@ -49,7 +50,7 @@ public class FolderStorage implements AutoCloseable {
 		this.maxFilesPerFolder = maxFilesPerFolder;
 		init();
 		initListingFileIfNotExists();
-		listingFile = new RandomAccessFile(listingFilePath.toFile(), "rws");
+		listingFile = new RandomAccessFile(listingFilePath.toFile(), "rw");
 	}
 
 	@Override
@@ -75,6 +76,7 @@ public class FolderStorage implements AutoCloseable {
 
 	public ListingFileEntry insert(final String filenamePrefix, final String filenameSuffix) throws IOException {
 
+		final long start = System.nanoTime();
 		ensureCapacity();
 
 		String filename = filenamePrefix + "$" + filenameSuffix;
@@ -88,6 +90,7 @@ public class FolderStorage implements AutoCloseable {
 		filesInSecondLevel++;
 
 		final ListingFileEntry result = updateListingFile(newFile);
+		METRICS_INSERT.debug("{}ms", (System.nanoTime() - start) / 1_000_000.0);
 
 		return result;
 	}
@@ -95,16 +98,13 @@ public class FolderStorage implements AutoCloseable {
 	private synchronized ListingFileEntry updateListingFile(final Path newFile) throws IOException {
 		final long offsetInListingFile = Files.size(listingFilePath);
 		// remember: all paths within storageBaseDirectory use only ascii characters
-		try (Writer out = Files.newBufferedWriter(listingFilePath, StandardCharsets.US_ASCII, StandardOpenOption.CREATE,
-				StandardOpenOption.APPEND, StandardOpenOption.SYNC)) {
 
-			final Path relativePath = storageBaseDirectory.relativize(newFile);
-			listingFile.seek(offsetInListingFile);
-			listingFile.write(relativePath.toString().getBytes(StandardCharsets.US_ASCII));
-			listingFile.write(NEWLINE);
-		}
-		final String filename = newFile.getFileName().toString();
 		final Path relativePath = storageBaseDirectory.relativize(newFile);
+		listingFile.seek(offsetInListingFile);
+		listingFile.write(relativePath.toString().getBytes(StandardCharsets.US_ASCII));
+		listingFile.write(NEWLINE);
+
+		final String filename = newFile.getFileName().toString();
 		return new ListingFileEntry(filename, offsetInListingFile, relativePath);
 	}
 
@@ -141,7 +141,7 @@ public class FolderStorage implements AutoCloseable {
 			final long start = System.nanoTime();
 			LOGGER.info("listing file not found -> creating a new one");
 			createNewListingFile();
-			METRICS_CREATE_LISTING_FILE.info(((System.nanoTime() - start) / 1_000_000.0) + "ms");
+			METRICS_CREATE_LISTING_FILE.debug("{}ms", (System.nanoTime() - start) / 1_000_000.0);
 		}
 	}
 
@@ -180,7 +180,7 @@ public class FolderStorage implements AutoCloseable {
 		} catch (final IOException e) {
 			throw new RuntimeIOException(e);
 		} finally {
-			METRICS_GET_PATH_BY_OFFSET.debug(((System.nanoTime() - start) / 1_000_000.0) + "ms");
+			METRICS_GET_PATH_BY_OFFSET.debug("{}ms", (System.nanoTime() - start) / 1_000_000.0);
 		}
 
 	}