void postLogFlush() {
    if (enabled.get() && !isPaused()) {
      /* start token saved before the flush is now safe to be persisted */
      persistFileLock.lock();
      startTokenSafeToPersist = startTokenBeforeLogFlush;
      try {
        // PersistCleanupToken because startTokenSafeToPersist changed.
        pruneHardDeleteRecoveryRange();
        persistCleanupToken();
      } catch (Exception e) {
        logger.error("Failed to persist cleanup token", e);
      } finally {
        persistFileLock.unlock();
      }
    }
  }
This method will be called after the log is flushed.
In production, this will be called in the PersistentIndex's IndexPersistor thread.
ambry-store/src/main/java/com/github/ambry/store/HardDeleter.java