code: protected void createInvalidationFile() {
          final String methodName = "createInvalidationFile()";
          final File f = new File(invalidationFileName);
          final CacheOnDisk cod = this;
          traceDebug(methodName, "cacheName=" + this.cacheName + " valueSet=" + cod.valueSet.size());
          AccessController.doPrivileged(new PrivilegedAction() {
              public Object run() {
                  FileOutputStream fos = null;
                  ObjectOutputStream oos = null;
                  try {
                      fos = new FileOutputStream(f);
                      oos = new ObjectOutputStream(fos);
  
                      oos.writeInt(cod.valueSet.size());
                      Iterator it = valueSet.iterator();
                      while (it.hasNext()) {
                          Object entryId = it.next();
                          oos.writeObject(entryId);
                      }
                  } catch (Throwable t1) {
                      com.ibm.ws.ffdc.FFDCFilter.processException(t1, "com.ibm.ws.cache.CacheOnDisk.createInvalidationFile", "1106", cod);
                      traceDebug(methodName, "cacheName=" + cod.cacheName + "\nException: " + ExceptionUtility.getStackTrace(t1));
                  } finally {
                      try {
                          oos.close();
                          fos.close();
                      } catch (Throwable t2) {
                          com.ibm.ws.ffdc.FFDCFilter.processException(t2, "com.ibm.ws.cache.CacheOnDisk.createInvalidationFile", "1113", cod);
                          traceDebug(methodName, "cacheName=" + cod.cacheName + "\nException: " + ExceptionUtility.getStackTrace(t2));
                      }
                  }
  
                  return null;
              }
          });
      }
docs: Call this method to create invalidation file to offload the invalidation cache ids. When the server is restarted,
  the invalidation cache ids are read back. The LPBT will be called to remove these cache ids from the disk.