hadoopmaster主机上传文件出错： put: File /a.txt._COPYING_ could only be replicated to 0 nodes instead of minReplication (=1). There are 3 datanode(s) running and 3 node(s) are excluded in this operation.

刚开始装好hadoop的时候，namenode机上传文件没有错误，今天打开时突然不能上传文件，报错

put: File /a.txt._COPYING_ could only be replicated to 0 nodes instead of minReplication (=1).
There are 3 datanode(s) running and 3 node(s) are excluded in this operation.

上网查了一下，先把，nnamenode和datanode的删掉，然后再运行hadoop namenode -format.尝试还是报错，想了一下，我根本就没有更新namenode，所以这个方法是没有用的。后来琢磨了一会，感觉是防火墙的原因，导致无法开通22端口。就关闭防火墙

systemctl stop firewalld.service #停止firewall

systemctl disable firewalld.service #禁止firewall开机启动

再试，成功解决！