@Override
  public void toData(DataOutput out,
      SerializationContext context) throws IOException {
    super.toData(out, context);
    DataSerializer.writeHashSet(persistentIds, out);
  }
The response to a {@link PrepareBackupRequest}, {@link AbortBackupRequest}, or
{@link FinishBackupRequest}.
geode-core/src/main/java/org/apache/geode/internal/cache/backup/BackupResponse.java