启动不了，分别报错如下：
ERROR: child process failed, exited with error number 1
ERROR: child process failed, exited with error number 51
1，第一次尝试启动mongod，报错，取消执行，
ai-learning-mongo-1-[root@ai-learning-mongo-1 mongodb]# su - mongo
Last login: Tue Jan 7 16:40:11 CST 2020 on pts/1
[mongo@ai-learning-mongo-1 ~]$ /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf &
[1] 16477
[mongo@ai-learning-mongo-1 ~]$ about to fork child process, waiting until server is ready for connections.
forked process: 16479
ERROR: child process failed, exited with error number 1
To see additional information in this output, start without the "--fork" option.
^C
[1]+ Exit 1 /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
2，检查目录，发现配置文件权限不对，修改目录权限
[mongo@ai-learning-mongo-1 ~]$ cd /app/mongodb/
[mongo@ai-learning-mongo-1 mongodb]$ ll
total 12
drwxr-xr-x 2 mongo mongo 248 Feb 26 2019 bin
drwxr-xr-x 8 mongo mongo 295 Jan 7 16:39 db
-r-------- 1 mongo mongo 1024 Feb 26 2019 keyfile
drwxr-xr-x 2 mongo mongo 24 Jan 7 15:46 logs
-rw-r--r-- 1 root root 785 Feb 26 2019 mongod.conf
-rw-r--r-- 1 mongo mongo 6 Jan 7 16:39 mongod.pid
[mongo@ai-learning-mongo-1 mongodb]$ chown -R mongo:mongo mongod.conf
chown: changing ownership of ‘mongod.conf’: Operation not permitted
[mongo@ai-learning-mongo-1 mongodb]$ exit
logout
3，修改配置文件权限
ai-learning-mongo-1-[root@ai-learning-mongo-1 ~]# chown -R mongo:mongo /app/mongodb/mongod.conf
4，尝试重新启动，还是报一样的错，排查可能目录属主还有可能是其他用户，继续往下级目录查看，发现bin目录下还是没有改掉权限
[2020/1/7 16:47:59] [mongo@ai-learning-mongo-1 mongodb]$ cd bin/
[2020/1/7 16:48:00] [mongo@ai-learning-mongo-1 bin]$ ll
[2020/1/7 16:48:00] total 285444
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 10546472 Nov 14 2018 bsondump
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 7770 Nov 14 2018 install_compass
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 33750624 Nov 14 2018 mongo
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 59532264 Nov 14 2018 mongod
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 12848240 Nov 14 2018 mongodump
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 10884072 Nov 14 2018 mongoexport
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 10767240 Nov 14 2018 mongofiles
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 11024432 Nov 14 2018 mongoimport
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 59036976 Nov 14 2018 mongoperf
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 13955512 Nov 14 2018 mongoreplay
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 13941224 Nov 14 2018 mongorestore
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 34171112 Nov 14 2018 mongos
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 11080688 Nov 14 2018 mongostat
[2020/1/7 16:48:00] -rwxr-xr-x 1 root root 10721912 Nov 14 2018 mongotop
[2020/1/7 16:48:34] [mongo@ai-learning-mongo-1 bin]$ exit
[2020/1/7 16:48:34] logout
5，修改整个目录结构的权限
[2020/1/7 16:49:07] ai-learning-mongo-1-[root@ai-learning-mongo-1 mongodb]# chown -R mongo:mongo /app/mongodb/
6，尝试启动，报error number 51
[2020/1/7 17:09:53] [mongo@ai-learning-mongo-1 db]$ /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf &
[2020/1/7 17:09:53] [1] 18399
[2020/1/7 17:09:53] [mongo@ai-learning-mongo-1 db]$ about to fork child process, waiting until server is ready for connections.
[2020/1/7 17:09:53] forked process: 18401
[2020/1/7 17:09:56] ERROR: child process failed, exited with error number 51
[2020/1/7 17:09:56] To see additional information in this output, start without the "--fork" option.
[2020/1/7 17:10:41] ^C
[2020/1/7 17:10:41] [1]+ Exit 51 /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
7，启动不了mongodb,查看日志 less /app/mongodb/logs/mongod.log
8，根据报错，open： too many open files ,认为应该是open files 数量不够，查阅资料修改
https://blog.csdn.net/qq_26614295/article/details/81502338
根据以上文章修改ulimit -HSn 102400 参数，并且删除lock文件，清空log/目录下日志，
[2020/1/7 17:25:25] [mongo@ai-learning-mongo-1 db]$ rm -f mongod.lock
[2020/1/7 17:25:29] [mongo@ai-learning-mongo-1 mongodb]$ cd logs
[2020/1/7 17:25:30] [mongo@ai-learning-mongo-1 logs]$ ll
[2020/1/7 17:25:30] total 24
[2020/1/7 17:25:30] -rw-rw-r-- 1 mongo mongo 21997 Jan 7 17:09 mongod.log
[2020/1/7 17:25:55] [mongo@ai-learning-mongo-1 logs]$ mv mongod.log /tmp/mongod.log.25
9，重启启动，成功启动mongod
[2020/1/7 17:26:09] [mongo@ai-learning-mongo-1 logs]$ /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf &
[2020/1/7 17:26:09] [1] 19549
[2020/1/7 17:26:09] [mongo@ai-learning-mongo-1 logs]$ about to fork child process, waiting until server is ready for connections.
[2020/1/7 17:26:09] forked process: 19552
[2020/1/7 17:26:21] child process started successfully, parent exiting
[2020/1/7 17:26:29]
[2020/1/7 17:26:29] [1]+ Done /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
[2020/1/7 17:26:35] [mongo@ai-learning-mongo-1 logs]$ ps -ef | grep mongod
[2020/1/7 17:26:35] appuser 13359 9538 0 16:01 pts/0 00:00:00 vi mongod.conf
[2020/1/7 17:26:35] mongo 19552 1 50 17:26 ? 00:00:13 /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
[2020/1/7 17:26:35] mongo 19679 19442 0 17:26 pts/1 00:00:00 grep --color=auto mongod
10，另外一台服务器修改以上参数，并且修改目录权限，重启发现报错error number 1
11，查看mongod.log ，尝试提高--maxConnes,启动失败，报错error number 1
[mongo@ai-learning-mongo-2 logs]$ /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf --maxConns=2000 &
12，根据日志钟第二行，permissions on /app/mongodb/keyfile are too open ,怀疑/app/mongodb/keyfile文件权限给多了，查看权限，果然给多了，收回权限改为400，
[mongo@ai-learning-mongo-2 logs]$ ll /app/mongodb/keyfile
[2020/1/7 17:39:47] -rwxrwxrwx 1 mongo mongo 1024 Feb 26 2019 /app/mongodb/keyfile
[2020/1/7 17:40:12] [mongo@ai-learning-mongo-2 logs]$ exit
[2020/1/7 17:40:12] logout
[2020/1/7 17:40:23] ai-learning-mongo-2-[root@ai-learning-mongo-2 mongodb]# chmod 400 /app/mongodb/keyfile
13，再次尝试启动，此次启动成功
[2020/1/7 17:41:14] ai-learning-mongo-2-[root@ai-learning-mongo-2 mongodb]# su - mongo
[2020/1/7 17:41:15] Last login: Tue Jan 7 17:33:56 CST 2020 on pts/2
[2020/1/7 17:41:27] [mongo@ai-learning-mongo-2 ~]$ /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf &
[2020/1/7 17:41:27] [1] 21252
[2020/1/7 17:41:27] [mongo@ai-learning-mongo-2 ~]$ about to fork child process, waiting until server is ready for connections.
[2020/1/7 17:41:27] forked process: 21254
[2020/1/7 17:41:34] child process started successfully, parent exiting
[2020/1/7 17:42:01]
[2020/1/7 17:42:01] [1]+ Done /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
[2020/1/7 17:42:01] [mongo@ai-learning-mongo-2 ~]$
[2020/1/7 17:42:07] [mongo@ai-learning-mongo-2 ~]$ ps -ef | grep mongo
[2020/1/7 17:42:07] root 21222 20426 0 17:41 pts/2 00:00:00 su - mongo
[2020/1/7 17:42:07] mongo 21223 21222 0 17:41 pts/2 00:00:00 -bash
[2020/1/7 17:42:07] mongo 21254 1 17 17:41 ? 00:00:06 /app/mongodb/bin/mongod --config /app/mongodb/mongod.conf
[2020/1/7 17:42:07] mongo 21389 21223 0 17:42 pts/2 00:00:00 ps -ef
[2020/1/7 17:42:07] mongo 21390 21223 0 17:42 pts/2 00:00:00 grep --color=auto mongo
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