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How to Change a MySQL Data Directory to a New Location on CentOS 7

Published on December 19, 2016
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By Melissa Anderson
Developer and author at DigitalOcean.
How to Change a MySQL Data Directory to a New Location on CentOS 7

Introduction

Databases grow over time, sometimes outgrowing the space on the file system. You can also run into I/O contention when they’re located on the same partition as the rest of the operating system. RAID, network block storage, and other devices can offer redundancy and other desirable features. Whether you’re adding more space, evaluating ways to optimize performance, or looking to take advantage of other storage features, this tutorial will guide you through relocating MySQL’s data directory.

Prerequisites

To complete this guide, you will need:

In this example, we’re moving the data to a block storage device mounted at /mnt/volume-nyc1-01. You can learn how to set one up in the How To Use Block Storage on DigitalOcean guide.

No matter what underlying storage you use, this guide can help you move the data directory to a new location.

Step 1 — Moving the MySQL Data Directory

To prepare for moving MySQL’s data directory, let’s verify the current location by starting an interactive MySQL session using the administrative credentials.

  1. mysql -u root -p

When prompted, supply the MySQL root password. Then from the MySQL prompt, select the data directory:

  1. select @@datadir;
Output
+-----------------+ | @@datadir | +-----------------+ | /var/lib/mysql/ | +-----------------+ 1 row in set (0.00 sec)

This output confirms that MySQL is configured to use the default data directory, /var/lib/mysql/, so that’s the directory we need to move. Once you’ve confirmed this, type exit and press “ENTER” to leave the monitor:

  1. exit

To ensure the integrity of the data, we’ll shut down MySQL before we actually make changes to the data directory:

  1. sudo systemctl stop mysqld

systemctl doesn’t display the outcome of all service management commands, so if you want to be sure you’ve succeeded, use the following command:

  1. sudo systemctl status mysqld

You can be sure it’s shut down if the final line of the output tells you the server is stopped:

Output
. . . Jul 18 11:24:20 ubuntu-512mb-nyc1-01 systemd[1]: Stopped MySQL Community Server.

Now that the server is shut down, we’ll copy the existing database directory to the new location with rsync. Using the -a flag preserves the permissions and other directory properties, while-v provides verbose output so you can follow the progress.

Note: Be sure there is no trailing slash on the directory, which may be added if you use tab completion. When there’s a trailing slash, rsync will dump the contents of the directory into the mount point instead of transferring it into a containing mysql directory:

  1. sudo rsync -av /var/lib/mysql /mnt/volume-nyc1-01

Once the rsync is complete, rename the current folder with a .bak extension and keep it until we’ve confirmed the move was successful. By re-naming it, we’ll avoid confusion that could arise from files in both the new and the old location:

  1. sudo mv /var/lib/mysql /var/lib/mysql.bak

Now we’re ready to turn our attention to configuration.

Step 2 — Pointing to the New Data Location

MySQL has several ways to override configuration values. By default, the datadir is set to /var/lib/mysql in the /etc/my.cnf file. Edit this file to reflect the new data directory:

  1. sudo vi /etc/my.cnf

Find the line in the [mysqld] block that begins with datadir=, which is separated from the block heading with several comments. Change the path which follows to reflect the new location. In addition, since the socket was previously located in the data directory, we’ll need to update it to the new location:

/etc/my.cnf
[mysqld]
. . .
datadir=/mnt/volume-nyc1-01/mysql
socket=/mnt/volume-nyc1-01/mysql/mysql.sock
. . .

After updating the existing lines, we’ll need to add configuration for the mysql client. Insert the following settings at the bottom of the file so it won’t split up directives in the [mysqld] block:

/etc/my.cnf
[client]
port=3306
socket=/mnt/volume-nyc1-01/mysql/mysql.sock

When you’re done, hit ESCAPE, then type :wq! to save and exit the file.

Step 3 — Restarting MySQL

Now that we’ve updated the configuration to use the new location, we’re ready to start MySQL and verify our work.

  1. sudo systemctl start mysqld
  2. sudo systemctl status mysqld

To make sure that the new data directory is indeed in use, start the MySQL monitor.

  1. mysql -u root -p

Look at the value for the data directory again:

  1. select @@datadir;
Output
+----------------------------+ | @@datadir | +----------------------------+ | /mnt/volume-nyc1-01/mysql/ | +----------------------------+ 1 row in set (0.01 sec)

Now that you’ve restarted MySQL and confirmed that it’s using the new location, take the opportunity to ensure that your database is fully functional. Once you’ve verified the integrity of any existing data, you can remove the backup data directory with sudo rm -Rf /var/lib/mysql.bak.

Conclusion

In this tutorial, we’ve moved MySQL’s data directory to a new location and updated SELinux to accommodate the adjustment. Although we were using a Block Storage device, the instructions here should be suitable for redefining the location of the data directory regardless of the underlying technology.

For more on managing MySQL’s data directories, see these sections in the official MySQL documentation:

If you’ve enjoyed this tutorial and our broader community, consider checking out our DigitalOcean products which can also help you achieve your development goals.

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The innodb_directories option, which defines directories to scan at startup for tablespace files, supports moving or restoring tablespace files to a new location while the server is offline. During startup, discovered tablespace files are used instead those referenced in the data dictionary, and the data dictionary is updated to reference the relocated files. If duplicate tablespace files are discovered by the scan, startup fails with an error indicating that multiple files were found for the same tablespace ID.

I was trying to change MySQL db directory but once I restarted the server it was showing port busy. I killed the process listening to that port, killed it and working perfectly.

For my server, I also need to update php.ini (/etc/php.ini) and restart apache: pdo_mysql.default_socket = /new/data/folder/mysql.sock mysql.default_socket = /new/data/folder/mysql.sock mysqli.default_socket = /new/data/folder/mysql.sock

run the below commands

  1. setenforce 0 and start the process
  2. sudo systemctl start mysqld

this will start the db

After following through the instructions I can’t start mysqld. I get an error that says, Job for mysql.service failed because the control process exited with error code. See “systemctl status mysql.service” and “journalctl -xe” for details.

While researching it, I found hte instructions for doing this on Ubuntu and it mentions needing to configure apparmor, which we don’t have in CentOS. That led me to looking a the permissions on the new location though. I tried sudo chown -R mysql:mysql on it with no luck. When I look at the location in the Files Browser I notice a padlock icon.

Thoughts?

In the conclusion you say, “we’ve moved MySQL’s data directory to a new location and updated SELinux to accommodate the adjustment” but there is no mention of SELinux anywhere in the article. Am I missing something?