This document discusses metadata locking in MySQL 5.5. It begins with an introduction to metadata locking and defines problems related to ALTER statements violating ACID properties, breaking replication, and causing deadlocks. It then covers how metadata locks work, how deadlock detection is implemented, how storage engines can integrate with metadata locking, and the performance impact of metadata locking. Finally, it outlines the roadmap for future improvements such as cross-storage-engine deadlock detection, consistent DDL across clusters, and sharding the metadata lock set.

1. Metadata locking in
MySQL 5.5
Konstantin Osipov
Engineer @ Mail.Ru
kostja.osipov@gmail.com
These slides released under the Creative Commons
Attribution­Noncommercial­Share Alike License

2. Mastering a 25-foot saw

3. DDL vs. DML: an illustration

4. What if... a transactional catalog

5. Locking agents

6. Define the problem (2)
Four groups of bugs:
- ALTER, DROP violate ACID
- … or ... break replication
- deadlocks in LOCK TABLES, FLUSH,
SET GLOBAL read_only and ALTER
- DML doesn't scale or THANK YOU
LOCK_open

7. Define the problem (3)

8. In come metadata locks

9. Deadlock detection
How does a typical wait-for grpah look like?

10. How to plug in
Engines need to plug-in to avoid cross-storage-
engine deadlocks.
- it's safe to #include “mdl.h”, no extra deps
- subclass MDL_waitfor_subgraph to accept
- MDL_waitfor_graph_visitor
- let us know when you're waiting, by calilng
- MDL_context::will_wait_for()

11. Performance impact

12. The roadmap
- help us add cross-storage-engine deadlock
detection!
- thr_lock.c and handler::store_lock() need not be
mandatory any more
- consistent DDL across all NDB cluster
- shard MDL_set (performance++)
- instrumentation

13. Performance impact

14. That's it.
Thank you for coming!
Further reading: WL#3873, WL#3726, WL#4284
Mdl.pdf is available in MySQL internals manual.