elasticsearch 进行snapshot nas

目前想通过snapshot备份索引数据到nas， 现在我在elasticSearch.ymll 添加如下配置：
path.repo= ["/mnt/backup"]

但是发现es无法启动  
 
我是使用本地路径模拟nas的， 
 
另外如果不使用本地， 怎么配置挂在外部存储呢 这个外部存储的路径怎么写呢
 
谢谢了！
 
目前创建仓库报错如下：

curl -XPUT 'http://172.20.2.101:9200/_snapshot/backup' -H 'Content-Type: application/json' -d '{ 

      "type":"fs", 

      "settings":{"location":"/mnt/backup","compress":true}

 }'

{"error":{"root_cause":[{"type":"repository_exception","reason":"[backup] location [/mnt/backup] doesn't match any of the locations specified by path.repo because this setting is empty"}],"type":"repository_exception","reason":"[backup] failed to create repository","caused_by":{"type":"repository_exception","reason":"[backup] location [/mnt/backup] doesn't match any of the locations specified by path.repo because this setting is empty"}},"status":500}

但是yml 添加path.repo配置后，发现无法启动了，去掉则启动成功。 还请大神帮忙看看。
 
elk 版本： 6.1.1