Hey there, let's work through this slow query log issue you're hitting. The errors you're seeing are a mix of file permissions and SELinux context problems—here's how to resolve them step by step:

1. Ensure the Log File Exists with Correct Basic Permissions #

First, make sure you're running these commands as root (use sudo if you're not logged in as root):

• Create the log file if it doesn't exist:

  touch /var/log/mysqld_slow.log
  

• Set the correct ownership to the MySQL user/group (this is critical—MySQL needs to read/write to this file):

  chown mysql:mysql /var/log/mysqld_slow.log
  

• Set secure file permissions (640 is sufficient; avoid 666 as it exposes the log to too many users):

  chmod 640 /var/log/mysqld_slow.log
  

2. Fix the SELinux Context Error #

The chcon: can't apply partial context to unlabeled file error happens because the new log file doesn't have an initial SELinux label. Here are two reliable fixes:

Option A: Restore Default Label First, Then Apply MySQL Log Context #

1. Restore the file's default SELinux label (this gives it a baseline context):

   restorecon -v /var/log/mysqld_slow.log
   

2. Now apply the MySQL log-specific context:

   chcon -t mysqld_log_t /var/log/mysqld_slow.log
   

Option B: Set a Persistent SELinux Rule (Recommended) #

The above fix might reset after a system reboot. To make the context permanent:

1. Add a persistent SELinux file context rule:

   semanage fcontext -a -t mysqld_log_t "/var/log/mysqld_slow.log"
   

2. Apply the rule to the file:

   restorecon -v /var/log/mysqld_slow.log
   

Verify the SELinux Context #

Run this command to confirm the context is set correctly:

ls -Z /var/log/mysqld_slow.log

You should see mysqld_log_t in the output.

3. Enable Slow Query Log (Temporary & Permanent) #

• Temporary enable (only lasts until MySQL restarts):

  set global slow_query_log = 'ON';
  

• Permanent enable (survives restarts):
  Edit your MySQL config file (usually /etc/my.cnf or /etc/mysql/my.cnf) and add these lines:

  slow_query_log = 1
  slow_query_log_file = /var/log/mysqld_slow.log
  long_query_time = 2  # Adjust this to your threshold (e.g., 2 seconds for slow queries)
  

  Then restart MySQL:

  systemctl restart mysqld
  

Alternative: Use MySQL's Default Log Directory #

If SELinux is still giving you trouble, consider placing the slow query log in MySQL's default data directory (typically /var/lib/mysql/). This directory already has the correct SELinux context for MySQL files:

set global slow_query_log_file = '/var/lib/mysql/mysqld_slow.log';
set global slow_query_log = 'ON';

For permanent setup, update the slow_query_log_file path in your MySQL config to this location.

内容的提问来源于stack exchange，提问作者Fred