private void doChannelFlushWithFailures(Oplog[] oplogs, int numFailures) throws IOException {
    nFakeChannelWrites = numFailures;
    ol = oplogs[0];
    ch = ol.getFileChannel();
    spyCh = spy(ch);
    ol.testSetCrfChannel(spyCh);

    byte[] entry1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19};
    byte[] entry2 = {100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115,
        116, 117, 118, 119};

    bb1 = ol.getWriteBuf();
    try {
      // Force channel.write() failures when writing the first entry
      doAnswer(new FakeChannelWriteBB()).when(spyCh).write(bb1);
      long chStartPos = ol.getFileChannel().position();
      bb1.clear();
      bb1.put(entry1);
      ol.flushAll(true);

      // Write the 2nd entry without forced channel failures
      nFakeChannelWrites = 0;
      bb1 = ol.getWriteBuf();
      bb1.clear();
      bb1.put(entry2);
      ol.flushAll(true);
      long chEndPos = ol.getFileChannel().position();
      assertEquals("Change in channel position does not equal the size of the data flushed",
          entry1.length + entry2.length, chEndPos - chStartPos);
      ByteBuffer dst = ByteBuffer.allocateDirect(entry1.length);
      ol.getFileChannel().position(chStartPos);
      ol.getFileChannel().read(dst);
      verifyBB(dst, entry1);
    } finally {
      region.destroyRegion();
    }
  }
This method tries to write half of the byte buffer to the channel.
geode-core/src/integrationTest/java/org/apache/geode/internal/cache/OplogFlushTest.java