MySQL Blog Archive
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Feature Preview - Multi-Threaded Replication Slaves

This week, MySQL released a feature-preview – the ability to have multiple threads on the slave so that it is better able to keep pace with the updates being applied on the master. To simplify potential sequencing issues, all updates for a particular database will be handled by the same thread – in other words you need to make sure that your application uses multiple databases in order to see the benefits.
Luís Soares (from the development team) has written a great Blog going into the details.