MariaDB 10 CentOS 7 moving datadir woes

Question

Brand new \"minimal\" install of CentOS 7 along with MariaDB 10. I have an additional mounted mirrored volume that I want to use for the datadir. Startup sequence is fine and co

Answer 1

I found this step by step guide working for me: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/SELinux_Users_and_Administrators_Guide/sect-Managing_Confined_Services-MariaDB-Configuration_Examples.html

You must install: yum install policycoreutils-python

Guide:

View the SELinux context of the default database location for mysql:

~]# ls -lZ /var/lib/mysql
drwx------. mysql mysql system_u:object_r:mysqld_db_t:s0 mysql

This shows mysqld_db_t which is the default context element for the location of database files. This context will have to be manually applied to the new database location that will be used in this example in order for it to function properly.

Stop the mysqld daemon:

~]# systemctl stop mariadb.service

Create a new directory for the new location of the database(s). In this example, /mysql/ is used:

~]# mkdir -p /mysql

Copy the database files from the old location to the new location:

~]# cp -R /var/lib/mysql/* /mysql/

Change the ownership of this location to allow access by the mysql user and group. This sets the traditional Unix permissions which SELinux will still observe:

~]# chown -R mysql:mysql /mysql

Run the following command to see the initial context of the new directory:

~]# ls -lZ /mysql
drwxr-xr-x. mysql mysql unconfined_u:object_r:usr_t:s0   mysql

The context usr_t of this newly created directory is not currently suitable to SELinux as a location for MariaDB database files. Once the context has been changed, MariaDB will be able to function properly in this area.

Open the main MariaDB configuration file /etc/my.cnf with a text editor and modify the datadir option so that it refers to the new location. In this example the value that should be entered is /mysql:

[mysqld]
datadir=/mysql

Save this file and exit.

Start mysqld. The service should fail to start, and a denial message will be logged to the /var/log/messages file:

~]# systemctl start mariadb.service

Job for mariadb.service failed. See 'systemctl status postgresql.service' and 'journalctl -xn' for details.

However, if the audit daemon is running and with him the setroubleshoot service, the denial will be logged to the /var/log/audit/audit.log file instead: SELinux is preventing /usr/libexec/mysqld "write" access on /mysql. For complete SELinux messages. run sealert -l b3f01aff-7fa6-4ebe-ad46-abaef6f8ad71

The reason for this denial is that /mysql/ is not labeled correctly for MariaDB data files. SELinux is stopping MariaDB from having access to the content labeled as usr_t. Perform the following steps to resolve this problem:

Run the following command to add a context mapping for /mysql/. Note that the semanageutility is not installed by default. If it missing on your system, install the policycoreutils-pythonpackage.

**~]# semanage fcontext -a -t mysqld_db_t "/mysql(/.*)?"**

This mapping is written to the /etc/selinux/targeted/contexts/files/file_contexts.local file:

~]# grep -i mysql /etc/selinux/targeted/contexts/files/file_contexts.local

/mysql(/.*)? system_u:object_r:mysqld_db_t:s0

Now use the restorecon utility to apply this context mapping to the running system:

**~]# restorecon -R -v /mysql**

Now that the /mysql/ location has been labeled with the correct context for MariaDB, mysqldstarts:

~]# systemctl start mariadb.service

Confirm the context has changed for /mysql/:

~]$ ls -lZ /mysql
drwxr-xr-x. mysql mysql system_u:object_r:mysqld_db_t:s0 mysql

The location has been changed and labeled, and mysqld has started successfully. At this point all running services should be tested to confirm normal operation.

Answer 2

The issue is indeed SELinux; you need to do three things before MariaDB / MySQL will start on CentOS 7:

1. Ensure the user:group is mysql:mysql
2. Set the SELinux tag to mysqld_db_t
3. Set the SELinux user to system_u

This is as simple as:

chcon -Rt mysqld_db_t /database/db
chcon -Ru system_u /database/db
chown -R mysql:mysql /database/db

The whole thing I needed to do after plugging in a disk is below:

cfdisk /dev/sdb
pvcreate /dev/sdb1
vgcreate database /dev/sdb1
lvcreate -l 100%FREE -n db database
mkfs.ext4 /dev/database/db
mkdir /database
mount /database
mkdir /database/db
chcon -Rt mysqld_db_t /database/db
chcon -Ru system_u /database/db
chown -R mysql:mysql /database/db
systemctl start mariadb

Answer 3

Well that was interesting...

It turns out, that CentOS 7 "minimal" installs SELinux, which apparently was preventing mysql from writing to the mounted mirrored set. I was looking for security items that I might not have thought about and found it right there in the docs. I had previously thought (obviously erroneously) that SELinux was a distribution, not a module. Once I ran the test to see if it was there....

getenforce

I temporarily disabled it to test.

setenforce 0

Finally, I was able to start MariaDB with the directory in the mirrored set as the datadir and no errors. To make this change permanent (because this server is behind a firewall), in /etc/selinux/config, I made

- SELINUX=enforcing
+ SELINUX=disabled

I hope this helps someone else. Have a great day!