Nova Resource:Deployment-prep/Databases

The beta cluster currently (May 2021) has a pair of MariaDB instances in a fairly standard master-replica setup running Buster and MariaDB 10.4.

Creating new instances

Derived from Setting up a MySQL replica and lessons learned from task T216067, slightly adjusted from task T276968. Still may be out of date, the last phabricator ticket may be useful.

• Create a new g3.cores8.ram16.disk20 instance in horizon named deployment-dbNN where NN is an unused number, add a large enough Cinder volume and mount it to /srv, get its puppet certs signed, etc.
  • TODO: determine if the VM needs that many resources. We used cores8.ram16 on g2 since we needed the extra disk, Cinder is now a thing.

On New Replica

• set mariadb::config::basedir: /opt/wmf-mariadb104 in hiera, adjust the version number based on the used MariaDB version
• ensure puppet runs cleanly
• /opt/wmf-mariadb104/scripts/mysql_install_db --user=mysql --basedir=/opt/wmf-mariadb104 --datadir=/srv/sqldata
• cd /srv/sqldata
• nc -l -p 9210 | mbstream -x

On Existing DB

Run mariabackup/innobackupex

• mariabackup --innobackupex --stream=xbstream /srv/sqldata --user=root --host=127.0.0.1 --slave-info | nc NEW-SERVER 9210

On New replica, After mariabackup

Do this before starting mysql!

• mariabackup --innobackupex --apply-log --use-memory=10G /srv/sqldata
• chown -R mysql: /srv
• systemctl start mariadb
  • tail the error log in /srv/sqldata and check for any errors
• If this is newer version of MariaDB, run /opt/wmf-mariadb104/bin/mysql_upgrade --host=127.0.0.1 to ensure compatibility.

Starting Replication

• Check /srv/sqldata/xtrabackup_slave_info on the new host, it provides a partial CHANGE MASTER statement based on whichever host the target was replicating from. If this is correct, use this, adding MASTER_HOST and the repl account/password portions that are missing.
  • e.g. CHANGE MASTER to MASTER_USER='repl', MASTER_PASSWORD='...', MASTER_PORT=3306, MASTER_HOST='deployment-db1', MASTER_LOG_FILE='deployment-db1-bin.000026', MASTER_LOG_POS=191406295;
  • The master repl password should be in the file /var/lib/git/labs/private/modules/secret/secrets/mysql/repl_password on the deployment-puppetmaster (currently deployment-puppetmaster04 )
• If you ran the hot backup from the master, use the information in /srv/sqldata/xtrabackup_binlog_info as slave_info won't exists.
• If you copied from the secondary master, xtrabackup_slave_info will point to the active master; use xtrabackup_binlog_info instead.

Read only

Per phab:T110115 all database servers default to read only when they start. This is expected for the replicas, but the primary server needs to be set to read-write after making sure it is ok (nothing worrying in logs, replication looks fine). This can be performed by sshing into the current primary which can be found from mediawiki-config repo.

Remember to !log any actions on #wikimedia-releng ^connect and check before setting it to read only if someone is doing maintenance which expects it to be read only, on the same IRC channel.

taavi@deployment-db07:~$ sudo -i mysql
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 329
Server version: 10.4.18-MariaDB-log MariaDB Server

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

root@BETA[(none)]> select @@read_only;
+-------------+
| @@read_only |
+-------------+
|           1 |
+-------------+
1 row in set (0.000 sec)

root@BETA[(none)]> set global read_only = false;
Query OK, 0 rows affected (0.000 sec)

root@BETA[(none)]> select @@read_only;
+-------------+
| @@read_only |
+-------------+
|           0 |
+-------------+
1 row in set (0.000 sec)