Locks¶

As any backup tool, mydumper needs to be able to take a consistent backups. However, this is not an easy tasks if you have to sync multiple threads.

Synchronizing threads¶

When you take a consistent backup, you must set the point in time where the backup is going to be taken. If you use multiple threads to take the backup, all of them must to be in the same point in time. By default, mydumper uses the simplest way, which is send a FLUSH TABLE WITH READ LOCK, but there are other options.

Lock all Tables¶

In cases where FLUSH TABLE WITH READ LOCK is not available, we can use --lock-all-tables on mydumper to lock all the tables that we are going to export.

No Locks¶

If you don't want to use FLUSH TABLE WITH READ LOCK and LOCK TABLE, you can simple disable it using --no-locks which might cause a inconsistent backup. This option has been modified for Percona Server, as after sending START TRANSACTION WITH CONSISTENT SNAPSHOT, we are able to check if all the threads are in the same point in time and if it is not it will retry 3 more times. If it is not able to sync all the threads, it will continue informing that it will be an inconsistent backup.

Lock strategy¶

Savepoints¶

It is possible to use savepoints to reduce metadata locking issues.

Less Locking¶

In cases where you have non transcational tables, you can use this option to reduce the locking on transactional tables.

InnoDB tables only¶

In the cases where you are exporting transactional tables only, you should be using --trx-consistency-only. mydumper holds a lock to detect if the tables to dump are transactional, with this options we let it know that it can release it before reduce bloking scenarios.

TiDB Snapshot¶

In case that you are taking a backup from TiDB, --tidb-snapshot allows you set the snapsnot to be used.

DDL Locks¶

There are scenarios where a backup is cancel due DDL operations over tables that we are exporting. In order to avoid cancelling the backup, mydumper send DDL locks like LOCK INSTANCE FOR BACKUP or BACKUP STAGE START when possible.

Disable DDL locks¶

However, if you are tacking a partial backup or when you know that the operations that are being blocked are safe, you can disable this useing --skip-ddl-locks

Disable Percona Backup Locks¶

On Percona Server 5.7, we have the option LOCK TABLES FOR BACKUP which has been replace by LOCK INSTANCE FOR BACKUP on Percona Server 8.0. We can disable this DDL locking mechanisim with --no-backup-locks