最近在做一些Mysql的案例实验，结果发现了一个非常有趣的事情，就是在Mysql数据库datadir下创建一个目录，你猜会发生什么呢。

环境

Mysql数据库环境如下：

操作系统：CentOS Linux release 7.6.1810 (Core) 
Mysql版本：
[root@localhost] 15:27:47 [testdb]>select @@version;
+------------+
| @@version  |
+------------+
| 5.7.26-log |
+------------+
1 row in set (0.00 sec)

在Mysql数据库datadir下创建一个目录

检查数据库状态，在这里一定看仔细，有几个数据库

[root@localhost] 15:29:36 [testdb]>show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| heartbeat          |
| mysql              |
| performance_schema |
| pt                 |
| query_rewrite      |
| sys                |
| testdb             |
| testdb1            |
+--------------------+
9 rows in set (0.00 sec)

在Mysql数据库datadir目录下，手工创建一个tmp的目录

[mysql@mysql 3306]$ mkdir tmp
[mysql@mysql 3306]$ ll
total 12412
-rw-r-----. 1 mysql mysql       56 Jul 13 15:34 auto.cnf
-rw-------. 1 mysql mysql     1679 Jul 14 16:00 ca-key.pem
-rw-r--r--. 1 mysql mysql     1107 Jul 14 16:00 ca.pem
-rw-r--r--. 1 mysql mysql     1107 Jul 14 16:00 client-cert.pem
-rw-------. 1 mysql mysql     1675 Jul 14 16:00 client-key.pem
drwxr-x---. 2 mysql mysql       62 Jul 27 15:26 heartbeat
-rw-r-----. 1 mysql mysql      529 Sep 17 15:30 ib_buffer_pool
-rw-r-----. 1 mysql mysql 12582912 Sep 17 15:30 ibdata1
drwxr-x---. 2 mysql mysql     4096 Jul 13 15:34 mysql
-rw-r-----. 1 mysql mysql    55763 Aug 21 10:10 mysql.log
drwxr-x---. 2 mysql mysql     8192 Jul 13 15:34 performance_schema
-rw-------. 1 mysql mysql     1675 Jul 14 16:00 private_key.pem
drwxr-x---. 2 mysql mysql       62 Jul 14 15:55 pt
-rw-r--r--. 1 mysql mysql      451 Jul 14 16:00 public_key.pem
drwxr-x---. 2 mysql mysql       70 Aug 11 16:01 query_rewrite
-rw-r--r--. 1 mysql mysql     1107 Jul 14 16:00 server-cert.pem
-rw-------. 1 mysql mysql     1675 Jul 14 16:00 server-key.pem
drwxr-x---. 2 mysql mysql     8192 Jul 13 15:34 sys
drwxr-x---. 2 mysql mysql      103 Sep  9 15:23 testdb
drwxr-x---. 2 mysql mysql      170 Sep  7 17:54 testdb1
drwxrwxr-x. 2 mysql mysql        6 Sep 17 15:31 tmp

重启Mysql数据库实例

[mysql@mysql ~]$ ./mysql.sh 3306 stop
mysqladmin: [Warning] Using a password on the command line interface can be insecure.
(No info could be read for "-p": geteuid()=2001 but you should be root.)
Mysql stop is successfully. [  OK  ]
[mysql@mysql ~]$ 
[mysql@mysql ~]$ ./mysql.sh 3306 start
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Mysql start successfully. [  OK  

登录数据库，查看数据库状态

[mysql@mysql ~]$ sh mysql3306 
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.26-log MySQL Community Server (GPL)

Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

[root@localhost] 15:33:11 [(none)]>show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| heartbeat          |
| mysql              |
| performance_schema |
| pt                 |
| query_rewrite      |
| sys                |
| testdb             |
| testdb1            |
| tmp                |
+--------------------+
10 rows in set (0.00 sec)

看到没有，多了一个tmp的数据库，更有趣的是，这个tmp库还能里面创建表

[root@localhost] 15:34:13 [(none)]>use tmp;
Database changed
[root@localhost] 15:34:17 [tmp]>show tables;
Empty set (0.00 sec)

[root@localhost] 15:34:22 [tmp]>create table t_test (id int not null,name char(10) not null default '',primary key(id));
Query OK, 0 rows affected (0.03 sec)

[root@localhost] 15:34:52 [tmp]>show tables;
+---------------+
| Tables_in_tmp |
+---------------+
| t_test        |
+---------------+
1 row in set (0.00 sec)

总结

总结上面的实验结果，在这里给大家2个建议
建议一：不要随意在Mysql的datadir下创建备份目录或者临时目录
建议二：不要直接将Mysql的datadir设置成，操作系统的文件系统最上级目录，不然你的操作系统重启之后，你的数据库会多一个lost+found的一个数据库。