类似于如下情况：
Traceback (most recent call last):
File "python_del_cassandra.py", line 21, in <module>
d_time = K_devdata.execute(" select dtime from devicestatus where did = ‘%s‘ and dtime > %s and dtime < %s order by dtime asc ;" % (devsid,start,end))
File "/usr/lib64/python2.6/site-packages/cassandra/cluster.py", line 1594, in execute
result = future.result(timeout)
File "/usr/lib64/python2.6/site-packages/cassandra/cluster.py", line 3296, in result
raise self._final_exception
cassandra.Unavailable: code=1000 [Unavailable exception] message="Cannot achieve consistency level ONE" info={‘required_replicas‘: 1, ‘alive_replicas‘: 0, ‘consistency‘: ‘ONE‘}
repliction_factor=1,时，只要有一个节点DN（down）掉的话，就会报这样的错，如果把replication_factor 改为2以上的话就不会报这个错了。
replication_factor 即复制因子
To directly answer the question, replication factor (RF) controls the number of replicas of each data partition that exist in a cluster or data center (DC). In your case, you have 3 nodes and a RF of 1. That means that when a row is written to your cluster, that it is only stored on 1 node. This also means that your cluster cannot withstand the failure of a single node.
In contrast, consider a RF of 3 on a 3 node cluster. Such a cluster could withstand the failure of 1 or 2 nodes, and still be able to support queries for all of its data.
原文：http://www.cnblogs.com/xxdfly/p/5641613.html