我使用以下代码从cassandra查询数据：

from cassandra import ConsistencyLevel
from cassandra.cluster import Cluster
from cassandra.auth import PlainTextAuthProvider
from cassandra.query import SimpleStatement
import pandas as pd

cluster = Cluster(contact_points=['192.168.2.4'],port=9042)
session = cluster.connect()

def testContectRemoteDatabase():
    contact_points = ['192.168.2.4']
    auth_provider = PlainTextAuthProvider(username='XXX', password='XX')
    cluster = Cluster(contact_points=contact_points, auth_provider=auth_provider)
    session = cluster.connect()
    cql_str = 'select * from DB1.mytable ;'
    simple_statement = SimpleStatement(cql_str, consistency_level=ConsistencyLevel.ONE,fetch_size=2000000)
    execute_result = session.execute(simple_statement, timeout=None)
    result = execute_result._current_rows
    cluster.shutdown()
    df = pd.DataFrame(result)
    df.to_csv('./my_test.csv', index=False, mode='w', header=True)

if __name__ == '__main__':
    testContectRemoteDatabase()

当我设置 fetch_size=1000000 ，没有错误，但是当我设置 fetch_size=2000000 ，此错误消息是：

Traceback (most recent call last):
  File "test.py", line 24, in <module>
    testContectRemoteDatabase()
  File "test.py", line 17, in testContectRemoteDatabase
    execute_result = session.execute(simple_statement, timeout=None)
  File "cassandra\cluster.py", line 2618, in cassandra.cluster.Session.execute
  File "cassandra\cluster.py", line 4877, in cassandra.cluster.ResponseFuture.result
cassandra.cluster.NoHostAvailable: ('Unable to complete the operation against any hosts', {<Host: 192.168.2.4:9042 datacenter1>: ConnectionShutdown('errors=Connection heartbeat timeout after 30 seconds, last_host=192.168.2.4:9042')})

如何修复？