PdxInstance makePdxInstance() {
    final int DSCODE_SIZE = 1;
    final int LENGTH_SIZE = 4;
    final int PDX_TYPE_SIZE = 4;
    final int BYTES_TO_SKIP = DSCODE_SIZE + LENGTH_SIZE + PDX_TYPE_SIZE;
    ByteBuffer bb = os.toByteBuffer(BYTES_TO_SKIP);
    PdxType pt = newType;
    if (pt == null) {
      pt = existingType;
    }
    return new PdxInstanceImpl(pt, new PdxInputStream(bb), bb.limit());
  }
Make sure we have room to add a VLF offset.
geode-core/src/main/java/org/apache/geode/pdx/internal/PdxWriterImpl.java