kishoreg commented on a change in pull request #7301:
URL: https://github.com/apache/pinot/pull/7301#discussion_r689764206



##########
File path: 
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/store/SingleFileIndexDirectory.java
##########
@@ -327,51 +336,176 @@ private void persistIndexMap(IndexEntry entry)
       throws IOException {
     File mapFile = new File(_segmentDirectory, 
V1Constants.INDEX_MAP_FILE_NAME);
     try (PrintWriter writer = new PrintWriter(new BufferedWriter(new 
FileWriter(mapFile, true)))) {
-      String startKey = getKey(entry.key.name, entry.key.type.getIndexName(), 
true);
-
-      StringBuilder sb = new StringBuilder();
-      sb.append(startKey).append(" = ").append(entry.startOffset);
-      writer.println(sb.toString());
-
-      String endKey = getKey(entry.key.name, entry.key.type.getIndexName(), 
false);
-      sb = new StringBuilder();
-      sb.append(endKey).append(" = ").append(entry.size);
-      writer.println(sb.toString());
+      persistIndexMap(entry, writer);
     }
   }
 
-  private String getKey(String column, String indexName, boolean 
isStartOffset) {
-    return column + MAP_KEY_SEPARATOR + indexName + MAP_KEY_SEPARATOR + 
(isStartOffset ? "startOffset" : "size");
-  }
-
   private String allocationContext(IndexKey key) {
     return this.getClass().getSimpleName() + key.toString();
   }
 
+  /**
+   * This method sweeps the indices marked for removal. Exception is simply 
bubbled up w/o
+   * trying to recover disk states from failure. This method is expected to 
run during segment
+   * reloading, which has failure handling by creating a backup folder before 
conduct reloading.
+   */
+  private void cleanupRemovedIndices()
+      throws IOException {
+    if (!_shouldCleanupRemovedIndices) {
+      return;
+    }
+
+    // To keep track of indices to be retained and put them together
+    // compactly in the new index file.
+    long nextOffset = 0;
+    List<IndexEntry> retained = new ArrayList<>();
+    File tmpIdxFile = new File(_segmentDirectory, V1Constants.INDEX_FILE_NAME 
+ ".tmp");
+
+    // With FileChannel, we can seek to the data flexibly.
+    try (FileChannel srcCh = new RandomAccessFile(_indexFile, 
"r").getChannel();

Review comment:
       That's ok. Order is not really important. If we really want to, we can 
try to make the order deterministic by doing two passes over the old index 
entries and new index entries.




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