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Schema changes in MySQL for OpenStack Trove users

 | November 24, 2014 |  Posted In: MySQL, OpenStack

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People using OpenStack Trove instances can hit a common issue in the MySQL world: how to perform schema change operations while minimizing the impact on the database server? Let’s explore the options that can allow online schema changes.

Summary

Schema changes in MySQL for OpenStack Trove usersWith MySQL 5.5, pt-online-schema-change from Percona Toolkit is your best option for large tables while regular ALTER TABLE statements are only acceptable for small tables. Also beware of metadata locks.

With MySQL 5.6, almost all types of schema changes can be done online. Metadata locks can also be an issue. pt-online-schema-change can still be worth using as it is also online on read replicas.

Regular ALTER TABLE with MySQL 5.5

If you are still using MySQL 5.5, almost all schema changes will require a table rebuild and MySQL will set a write lock. Therefore all writes to the table that gets modified will be blocked. As soon as the table gets large or if you cannot afford to have a maintenance window, using ALTER TABLE becomes tricky.

The only exception is that secondary indexes can be added or removed without rebuilding the whole table. The table is still write locked during the operation but it is much faster.

You can spot this ‘fast index creation’ process by looking at SHOW PROCESSLIST (see manage keys in the State field):

It is helpful for small or even medium tables, but it is not fast enough for large tables.

A common workaround when you have a master-slave configuration is to perform the schema change on the slave first, promote the slave and then perform the schema change on the old master. However replication is not only available if you are using Icehouse and even with Juno, replicas are read-only. So this workaround is not usable with Trove.

pt-online-schema-change

A good alternative is to use pt-online-schema-change. In a nutshell the tool creates a new table with the desired structure, creates triggers on the original table, and copies rows in chunks from the original table to the new table.

Nice features of the tool include being able to write to the original table while the schema change is being performed (hence the name of the tool), monitoring replication lag on slaves and automatic throttling to keep lag under a reasonable threshold.

Let’s assume you want to add a new field called ‘city’ in the ‘user’ table. Instead of running:

you will run:

All ALTER TABLE statements can be converted to an equivalent pt-online-schema-change command.

Is there any tradeoff? The first one is that pt-online-schema-change will be slower than a plain ALTER TABLE. This is expected as the tool has extra code to throttle itself if it adds too much load.

How much slower is it? As always it depends. On a lightly loaded server the difference will be minimal, for instance I measured 3mn24s for ALTER TABLE and 3mn30s for pt-online-schema-change on a test table. However on a heavily loaded server, it can be like 5x slower.

The second tradeoff is that pt-online-schema-change adds triggers on the original table to capture data change. So if you already have triggers, you can’t use the tool (this will be lifted in MySQL 5.7).

Metadata Locks

Metadata locks were introduced in MySQL 5.5 for better transaction isolation.

But one side effect is: if you start a schema change operation on a table while another transaction is reading or writing on the same table, ALTER TABLE will be stuck in the Waiting for metadata lock state.

Another negative side effect of metadata locks is that in some situations ALTER TABLE will also block all queries to the table that is being altered, even reads. In the worst cases, this may create a query pileup situation that will freeze the whole server. For more on this topic, have a look at this post.

What about pt-online-schema-change and metadata locks? It is of course hit by this issue/feature. However the good thing is that the timeout for metadata locks is set by the tool to 60s instead of the default 1 year. So after 60s, pt-online-schema-change will simply retry the operation that failed because of metadata locks.

MySQL 5.6: Online Schema Changes?

Metadata locks also exist with MySQL 5.6, the same issues as described above can then happen.

However the good news with MySQL 5.6 is that most schema changes can be done online. Now the question is: should you use pt-online-schema-change or a regular online ALTER TABLE statement?

Both have pros and cons:

  • ALTER TABLE is easy to use while being confident with pt-online-schema-change requires some time.
  • There is no way for ALTER TABLE to know if it’s overloading a replica while pt-online-schema-change monitors replication lag and throttles itself if needed.
  • ALTER TABLE is only online for the master, not for replicas while pt-online-schema-change is online for all servers.

The last point can be a bit confusing: why an online ALTER TABLE on the master wouldn’t be online on the slave as well? The truth is that unless you are using multi-threaded replication, writes on slaves are serialized. So while the ALTER TABLE is running, the slave will not process any other write, which is not much different from an offline ALTER TABLE.

So if you are running a single server (all Icehouse users for instance), ALTER TABLE is probably the right choice. But if you have read replicas and specifically if the application cannot tolerate replication lag, pt-online-schema-change is definitely a better choice.

Conclusion

Performing schema changes is becoming easier with newer versions of MySQL. However potential issues can be found with any version, so always be careful when you need to run ALTER TABLE.

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Stephane Combaudon

Stéphane joined Percona in July 2012, after working as a MySQL DBA for leading French companies such as Dailymotion and France Telecom. In real life, he lives in Paris with his wife and their twin daughters. When not in front of a computer or not spending time with his family, he likes playing chess and hiking.

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