@Override
        public String explainFailure() throws Exception {
          long memstoreSize = desiredRegion.getMemStoreDataSize();
          if (memstoreSize > 0) {
            return "Still have unflushed entries in memstore, memstore size is " + memstoreSize;
          }
          return "Unknown";
        }
When a log roll is about to happen, we do a flush of the regions who will be affected by the
log roll. These flushes cannot be a selective flushes, otherwise we cannot roll the logs. This
test ensures that we do a full-flush in that scenario.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestPerColumnFamilyFlush.java