@Test
  public void testOverflowOplogByteArrayFlush() throws Exception {
    DiskRegionProperties props = new DiskRegionProperties();
    props.setOverFlowCapacity(1);
    region = DiskRegionHelperFactory.getSyncOverFlowOnlyRegion(cache, props);
    region.put("K1", "v1");
    region.put("K2", "v2");

    DiskRegion dr = ((LocalRegion) region).getDiskRegion();
    OverflowOplog oplog = dr.getDiskStore().overflowOplogs.getActiveOverflowOplog();
    assertNotNull("Unexpected null Oplog", oplog);

    doPartialChannelByteArrayFlushForOverflowOpLog(oplog);
  }
This method tries to write half of the byte buffer to the channel.
geode-core/src/integrationTest/java/org/apache/geode/internal/cache/OverflowOplogFlushTest.java