elasticsearch启动问题汇总
问题出现环境，OS版本：CentOS-7-x86_64-Minimal-1708；ES版本：elasticsearch-6.2.2。
1、max file descriptors [4096] for elasticsearch process is too low, increase to at least [65536]
每个进程最大同时打开文件数太小，可通过下面2个命令查看当前数量
ulimit -Hn ulimit -Sn
修改/etc/security/limits.conf文件，增加配置，用户退出后重新登录生效
* soft nofile 65536 * hard nofile 65536
2、max number of threads [3818] for user [es] is too low, increase to at least [4096]
问题同上，最大线程个数太低。修改配置文件/etc/security/limits.conf（和问题1是一个文件），增加配置
* soft nproc 4096 * hard nproc 4096
可通过命令查看
ulimit -Hu ulimit -Su
修改后的文件：
3、max virtual memory areas vm.max_map_count [65530] is too low, increase to at least [262144]
修改/etc/sysctl.conf文件，增加配置vm.max_map_count=262144
vi /etc/sysctl.conf
sysctl -p
执行命令sysctl -p生效
4、Exception in thread "main" java.nio.file.AccessDeniedException: /usr/local/elasticsearch/elasticsearch-6.2.2-1/config/jvm.options
elasticsearch用户没有该文件夹的权限，执行命令
chown -R es:es /usr/local/elasticsearch/