CentOS 7.8 MySQL启动失败 /usr/sbin/mysqld: Can‘t create/write to file ‘/var/run/mysqld/mysqld.pid‘

一、问题描述：

CentOS 7.8机器重启后 MySQL启动失败
[ERROR] /usr/sbin/mysqld: Can’t create/write to file ‘/var/run/mysqld/mysqld.pid’ (Errcode: 2 - No such file or directory)

注：本场景使用的是MySQL RPM安装包，版本号MySQL 5.7.12，目前MySQL 5.7的最新补丁版本为5.7.31，建议直接下载mysql-5.7.31-1.el7.x86_64.rpm-bundle.tar。

二、原因分析：

linux目录详细介绍

/var/run目录指向/run目录，而/run是一个临时文件系统，存储系统启动以来的信息。当系统重启时，这个目录下的文件应该被删掉或清除。
可想而知，CentOS重启后，/var/run/mysqld目录也将消失，而mysql5.7的启动脚本中未自动创建该目录，所以会提示该错误！

三、临时解决办法：

cd /var/run
mkdir mysqld
chown mysql:mysql mysqld
systemctl start mysqld

四、永久解决办法：

• 方法1：修改启动脚本，启动时若找不到/var/run/mysqld目录，则自动创建。（推荐）

vim /etc/init.d/mysqld

# 以下截取修改部分内容
mypiddir="/var/run/mysqld"
get_mysql_option mysqld_safe pid-file "$mypiddir/mysqld.pid"
mypidfile="$result"

#############################省略#############################

start(){
    
    
    [ -x $exec ] || exit 5
    # check to see if it's already running
    RESPONSE=$(/usr/bin/mysqladmin --no-defaults --socket="$adminsocket" --user=UNKNOWN_MYSQL_USER ping 2>&1)
    if [ $? = 0 ]; then
        # already running, do nothing
        action $"Starting $prog: " /bin/true
        ret=0
    elif echo "$RESPONSE" | grep -q "Access denied for user"
    then
        # already running, do nothing
        action $"Starting $prog: " /bin/true
        ret=0
    else
    	# 不存在/var/run/mysqld目录，则自动创建
        if [ ! -e "$mypiddir" -a ! -h "$mypiddir" ]
        then
           mkdir -p "$mypiddir" || exit 1
        fi
        chown mysql:mysql "$mypiddir"

        # prepare for start
        touch "$errlogfile"
        chown mysql:mysql "$errlogfile"

#############################省略#############################

systemctl daemon-reload
systemctl start mysqld

• 方法2：修改/etc/my.cnf配置文件,修改mysqld_safe属性组下pid-file文件位置

# For advice on how to change settings please see
# http://dev.mysql.com/doc/refman/5.7/en/server-configuration-defaults.html

[mysqld]
#
# Remove leading # and set to the amount of RAM for the most important data
# cache in MySQL. Start at 70% of total RAM for dedicated server, else 10%.
# innodb_buffer_pool_size = 128M
#
# Remove leading # to turn on a very important data integrity option: logging
# changes to the binary log between backups.
# log_bin
#
# Remove leading # to set options mainly useful for reporting servers.
# The server defaults are faster for transactions and fast SELECTs.
# Adjust sizes as needed, experiment to find the optimal values.
# join_buffer_size = 128M
# sort_buffer_size = 2M
# read_rnd_buffer_size = 2M
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
skip-name-resolve

# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0

sql-mode=ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

[mysqld_safe]
log-error=/var/log/mysqld.log
# 可改为/var/run/mysqld.pid
pid-file=/var/run/mysqld/mysqld.pid

systemctl start mysqld

说明：测试中发现，修改pid-file位置，确实可以启动成功，而且服务可正常使用，但是mysqld_safe守护进程无法获取mysql的运行状态，启动脚本一直处于等待状态（原因是mysqld_safe守护进程运行状态监测的对象仍然是原先的/var/run/mysqld/mysqld.pid）！
推测MySQL 5.7.12运行状态监测脚本可能有bug，暂不推荐此种方法。

如果大家有更好的解决办法，也可留言说明，非常感谢！