MySQL 5.6: Data dictionary LRU

Note: this article was originally published on http://blogs.innodb.com on April 8, 2011 by Sunny Bains.

In MySQL 5.6 we’ve added a new feature that closes and unloads table instances from the InnoDB internal data dictionary, once a user configurable threshold is reached. This ends the situation where you could have hundreds of megabytes caching rarely used entries until the server was restarted and will be particularly appreciated by hosting and software as a service providers.

For this we’ve used an existing MySQL config variable table-definition-cache.  This cache limit is a soft limit. This means that if the user has more than table-definition-cache tables open then InnoDB will not force eviction of the table from the InnoDB data dictionary cache.

Users don’t really have to do anything to make use of this feature. It should pick up the existing setting (default or explicitly set) and just work. This feature should benefit users that have 1000s of tables and open a subset of tables only rarely. The LRU mechanism will eventually mark them for eviction and remove them from the data dictionary cache.

Other benefits

Some of the additional benefits of this change are internal to InnoDB and help in improving the design. We now have well defined interfaces for opening and closing tables. All internal sub-systems  need to use the same interface as the MySQL table handler. This has helped in loosening the tight coupling between various InnoDB sub-systems and the data dictionary. By loosening this tight coupling, it will help us in adding features to the InnoDB data dictionary more easily.

Future work

This feature is by no means 100% complete. Currently we don’t evict tables that are in a foreign key relationship, either reference or those that refer to another table. Handling of such tables is work in progress, so stay tuned.

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