我正在尝试将 mysql 程序的所有执行（所有表数据、索引等）移至 RAM 磁盘上，以进行性能实验，从我的常规 SSD 中排除文件 IO。
我首先像平常一样安装 mysql：sudo apt install mysql-server。程序mysqld开始，我可以正常创建数据库。
我还按照此站点上列出的说明创建了一个 RAMDisk：在 Ubuntu 上创建 RAMDisk在该位置/mnt/ramdisk并使用大小25G（我有64G可用的 RAM）。我可以写入该位置并观察到我的 RAM 开始按预期填满。
现在，我使用以下命令将 mysql 库目录复制到此 RAMDisk：
cp -rp /var/lib/mysql /mnt/ramdisk
并符号链接到它，这样我就不必更改文件datadir中的参数/etc/mysql/mysql.conf.d/mysqld.cnf。
mv mysql _mysql
ln -s /mnt/ramdisk/mysql mysql
我还更改了权限，以便 MySQL“应该”能够访问 RAMDisk：
root@HOSTNAME:/var/lib# ls -l | grep mysql
lrwxrwxrwx 1 mysql mysql 19 Feb 14 16:45 mysql -> /mnt/ramdisk/mysql/
drwx------ 6 mysql mysql 4096 Feb 14 17:06 _mysql
drwx------ 2 mysql mysql 4096 Feb 14 12:14 mysql-files
drwx------ 2 mysql mysql 4096 Feb 12 17:14 mysql-keyring
drwxr-xr-x 2 root root 4096 Jan 21 06:10 mysql-upgrade
和
root@HOSTNAME:/mnt# ls -l
total 0
drwxrwxrwt 3 mysql mysql 60 Feb 14 17:24 ramdisk
root@HOSTNAME:/mnt/ramdisk# ls -l
total 0
drwx------ 5 mysql mysql 380 Feb 14 16:41 mysql
但是，当我现在尝试重新启动该mysqld程序时，遇到以下错误：
USER@HOSTNAME:/$ sudo /etc/init.d/mysql restart
[sudo] password for USER:
[....] Restarting mysql (via systemctl): mysql.serviceJob for mysql.service failed because the control process exited with error code.
See "systemctl status mysql.service" and "journalctl -xe" for details.
failed!
研究两者systemctl status mysql.service并journalctl -xe不能更深入地了解问题：
USER@HOSTNAME:/$ systemctl status mysql.service
● mysql.service - MySQL Community Server
Loaded: loaded (/lib/systemd/system/mysql.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Fri 2020-02-14 17:30:55 PST; 1min 0s ago
Process: 19794 ExecStart=/usr/sbin/mysqld --daemonize --pid-file=/run/mysqld/mysqld.pid (code=exited, status=1/FAILURE)
Process: 19785 ExecStartPre=/usr/share/mysql/mysql-systemd-start pre (code=exited, status=0/SUCCESS)
Main PID: 17945 (code=exited, status=0/SUCCESS)
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Service hold-off time over, scheduling restart.
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Scheduled restart job, restart counter is at 5.
Feb 14 17:30:55 HOSTNAME systemd[1]: Stopped MySQL Community Server.
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Start request repeated too quickly.
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Failed with result 'exit-code'.
Feb 14 17:30:55 HOSTNAME systemd[1]: Failed to start MySQL Community Server.
对于journalctl -xe：
-- Unit mysql.service has begun starting up.
Feb 14 17:30:54 HOSTNAME audit[19792]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19792 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
Feb 14 17:30:54 HOSTNAME audit[19792]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19792 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
Feb 14 17:30:54 HOSTNAME audit[19792]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19792 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
Feb 14 17:30:54 HOSTNAME audit[19796]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19796 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=125 ouid=125
Feb 14 17:30:54 HOSTNAME audit[19796]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19796 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=125 ouid=125
Feb 14 17:30:54 HOSTNAME audit[19796]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19796 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=125 ouid=125
Feb 14 17:30:54 HOSTNAME audit[19796]: AVC apparmor="DENIED" operation="open" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/ibdata1" pid=19796 comm="mysqld" requested_mask="wr" denied_mask="wr" fsuid=125 ouid=125
Feb 14 17:30:55 HOSTNAME mysqld[19794]: Initialization of mysqld failed: 0
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Control process exited, code=exited status=1
Feb 14 17:30:55 HOSTNAME systemd[1]: mysql.service: Failed with result 'exit-code'.
Feb 14 17:30:55 HOSTNAME systemd[1]: Failed to start MySQL Community Server.
-- Subject: Unit mysql.service has failed
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
--
-- Unit mysql.service has failed.
答案1
事实证明，问题就在输出中摆在我面前journalctl -xe。
Feb 14 17:30:54 HOSTNAME audit[19792]: AVC apparmor="DENIED" operation="mknod" profile="/usr/sbin/mysqld" name="/mnt/ramdisk/mysql/HOSTNAME.lower-test" pid=19792 comm="mysqld" requested_mask="c" denied_mask="c" fsuid=0 ouid=0
更具体地说，apparmor=“已拒绝”部分。来自 AskUbuntu 上的这个答案：mysql文件夹重定位后Apparmor错误我能够解决我的问题。我将在下面添加相关详细信息：
编辑/etc/apparmor.d/usr.sbin.mysqld文件。删除/var/lib/mysql并添加新的 datadir 位置。就我而言是
# Allow data dir access
/mnt/ramdisk/mysql/ r,
/mnt/ramdisk/mysql/** rwk,
重新启动apparmor：service systemctl restart apparmor
重启mysql服务：service mysql start