public long getNumBlocksFailedToUncache() {
    return numBlocksFailedToUncache.longValue();
  }
@Test(timeout=60000)
  public void testUncacheUnknownBlock() throws Exception {
    // Create a file
    Path fileName = new Path("/testUncacheUnknownBlock");
    int fileLen = 4096;
    DFSTestUtil.createFile(fs, fileName, fileLen, (short)1, 0xFDFD);
    HdfsBlockLocation[] locs = (HdfsBlockLocation[])fs.getFileBlockLocations(
        fileName, 0, fileLen);

    // Try to uncache it without caching it first
    setHeartbeatResponse(uncacheBlocks(locs));

    GenericTestUtils.waitFor(new Supplier<Boolean>() {
      @Override
      public Boolean get() {
        return fsd.getNumBlocksFailedToUncache() > 0;
      }
    }, 100, 10000);
  }