public void ensureSingleLevel() throws IOException {
      if (numLevels > 1) {
        throw new IOException(
          "Wrote a " + numLevels + "-level index with " + rootChunk.getNumEntries()
            + " root-level entries, but " + "this is expected to be a single-level block index.");
      }
    }
@throws IOException if we happened to write a multi-level index.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlockIndex.java