The document discusses methods for performing schema changes in MySQL replication and Galera clusters, including rolling schema changes and online DDL. It highlights the limitations of schema changes, particularly in respect to replication, and reviews tools like pt-online-schema-change for executing these alterations with minimal downtime. Additionally, it provides guidance on handling different table sizes and scenarios for effective schema updates.

1. Copyright 2015 Severalnines AB
Schema Changes for MySQL Replication & Galera Cluster
August 25, 2015
Krzysztof Książek
Severalnines
krzysztof@severalnines.com
1

2. Copyright 2015 Severalnines AB
! We want to help all non-DBA people who look after
MySQL infrastructures
! Share tips and good practices
! Watch the replays of the previous webinars on our
Slideshare page
! http://www.slideshare.net/Severalnines/videos
2
“Become a MySQL DBA” series

3. Copyright 2015 Severalnines AB
! Different methods to perform schema changes on MySQL Replication
and Galera
! rolling schema change
! online alters
! external tools, e.g., pt-online-schema-change
! Differences between MySQL 5.5 and 5.6
! Differences between MySQL Replication vs Galera
! Example real-life scenarios with MySQL Replication and Galera setups
3
Agenda

4. Copyright 2015 Severalnines AB
Schema changes in MySQL
4

5. Copyright 2015 Severalnines AB
! Application development
! New features to existing
modules
! New modules
! Data management and
archiving (partitions)
! Schema optimization
! Performance optimization
5
Why schemas need changes?

6. Copyright 2015 Severalnines AB
! Point-in-time schema changes
! Everything happens at a given point of time, over the
whole infrastructure
! Rolling schema changes
! Changes happen at different times across the
infrastructure
! Requirement is that schema versions have to be
compatible with each other
6
Types of schema changes

7. Copyright 2015 Severalnines AB
! Schema changes cause significant performance
overhead as they are I/O-heavy operations
! Schema changes may take long time to finish on larger
tables
! Direct schema changes don’t play well with MySQL
replication due to the lag inducted
! Direct schema changes don’t play well with Galera
! Rolling schema changes may require significant amount
of work
7
Schema changes - limitations

8. Copyright 2015 Severalnines AB
! Replication has it’s limitations, only some changes can be
replicated. RBR is very strict here
! Remove a column at the end of the table - yes
! Remove a column in the middle - no go
! Add a column at the end - works
! Add a column in the middle - not really
! Not every conversion between column types works -
refer to the documentation
8
Schema changes - limitations

9. Copyright 2015 Severalnines AB
! You can’t change the
schema and the code at
the exact time
! New schema have to be
compatible with old code
! Depreciate columns, not
remove
! Remove them two, three
iterations later, once you
confirm you _can_ remove
9
Schema changes - limitations

10. Copyright 2015 Severalnines AB
Ways to perform a schema change
10

11. Copyright 2015 Severalnines AB
! Online DDL
! Rolling schema upgrade
! Online schema change
! Rolling Schema Upgrade for Galera
! Total Order Isolation for Galera
11
Methods to perform a schema change

12. Copyright 2015 Severalnines AB
! DDL, historically, was a locking operation
! Some of them were fast, i.e. DROP (secondary) INDEX
! InnoDB plugin in 5.1 and 5.5 introduced Fast Index
Creation that allowed reads to be executed while
secondary index has been added
! MySQL 5.6 introduced online DDL’s for InnoDB
12
Online DDL

13. Copyright 2015 Severalnines AB
! Many operations are non-blocking for reads and writes
! Add an index
! Add a column
! Reorder columns
! Add a primary key
! and many others (check the documentation)
! https://dev.mysql.com/doc/refman/5.6/en/innodb-create-
index-overview.html#innodb-online-ddl-summary-grid
13
Online DDL

14. Copyright 2015 Severalnines AB
! Busy tables may be an issue
(innodb_online_alter_log_max_size)
! Lag will be an issue - online DDL is online on the master
only
! Not suitable for large tables in replication setup
14
Online DDL

15. Copyright 2015 Severalnines AB
! Run DDL from the bottom to the top of replication chain
! SET SESSION sql_log_bin=0 to avoid errant transactions
! Once you reach the master, failover and then alter the
old master
! Schema changes have to be compatible, you’ll be
running DML’s on a mix of altered and not altered tables
! Schema changes have to be compatible, you’ll be
reading from a mix of altered and not altered tables
15
Rolling Schema Upgrade

16. Copyright 2015 Severalnines AB
! Different tools (pt-online-schema-change, Online Schema
Change from FB), same idea
! Create a new table with desired schema
! Setup triggers to copy data from old to the new table
! Copy the data from old to new table in batches
! Rename table once the data has been copied
16
Online Schema Change - external tools

17. Copyright 2015 Severalnines AB
Online Schema Change - external tools
17

18. Copyright 2015 Severalnines AB
! pt-online-schema-change - the most popular tool
! Point-in-time change, you can use for any DDL
! Replicaton-aware, it can monitor lag and throttle itself
! Introduces additional load, slow down your system
! It has issues with foreign keys
! Can’t work with tables which have triggers
! Won’t work with tables without PK and unique key
18
pt-online-schema-change

19. Copyright 2015 Severalnines AB
! Otherwise it’s a very reliable tool
! Schema change may take a long time but it’s throttling
itself and can be paused at any time
! One of the most important tools in the DBA arsenal
! Sometimes it’s the only way to run a schema change
without downtime
19
pt-online-schema-change

20. Copyright 2015 Severalnines AB
Schema changes in Galera Cluster
20

21. Copyright 2015 Severalnines AB
! All changes happen at the
exact time in the cluster
! All changes are blocking
(you won’t benefit from
online DDL’s)
! Great for consistency
! You can run any change
this way
! Not suitable for larger tables
due to severe locking
21
Total Order Isolation

22. Copyright 2015 Severalnines AB
! Similar to the rolling schema upgrade we discussed earlier
! Process is partially automated - you need to set
wsrep_OSU_method to RSU and proceed with DDL
! Node will be in Desync state for a duration of the DDL
! Once DDL finishes, node applies remaining writesets
! Writesets are stored in gcache, both in memory and on
disk
22
Rolling Schema Upgrade

23. Copyright 2015 Severalnines AB
Rolling Schema Upgrade
23

24. Copyright 2015 Severalnines AB
Different scenarios for a schema
change
24

25. Copyright 2015 Severalnines AB
! If a change is non-blocking - online DDL
! pt-online-schema-change may also work
! pt-osc should not take more than a couple of minutes
! For Galera - TOI if you can accept few seconds of stall
! Otherwise - pt-online-schema-change is the only option
25
Small tables (up to 10s of a direct DDL)

26. Copyright 2015 Severalnines AB
! pt-online-schema-change is the best option here for both
MySQL and Galera
! pt-online-schema-change may take even couple of hours
to finish
! It’s less cumbersome than rolling upgrade
! Rolling schema upgrade may be the option of choice if
pt-osc can’t be used due to it’s limitations (no PK, triggers
in altered table, etc.)
26
Medium tables (20 - 30m, up to 1h)

27. Copyright 2015 Severalnines AB
! pt-osc will work but it will take long time (36 - 48h) to finish
! You need to have enough disk space for binlogs and
altered copy of the table
! Rolling schema upgrade may be a feasible option
! For Galera pt-osc can be also the best option
! RSU is another option but ensure you have large gcache
(or a space on disk for gcache files)
27
Large tables (more than 1h, up to 12h)

28. Copyright 2015 Severalnines AB
! Are you sure that you need such a large table?
! Try to avoid them by splitting data across many tables or
archiving old data
! Rolling schema upgrade is the way to go
! Use snapshots or xtrabackup to provision new nodes
! Galera may need large gcache (both in memory and on
disk)
! Process of joining desynced node will take a long time
28
Very large tables (more than 12h)

29. Copyright 2015 Severalnines AB
! Schema change speed depends on your hardware and
data structure
! Test the changes on development environment
! Is it worth using triggers or foreign keys and not have the
option to use pt-osc?
! You _do_ want to have PK defined in all tables - do it
before it grows too large to alter
! Schema changes will happen, get used to them
29
What to keep in mind?

30. Copyright 2015 Severalnines AB
! More blogs in “Become a MySQL DBA” series:
! http://www.severalnines.com/blog/become-mysql-
dba-blog-series-query-tuning-process
! http://www.severalnines.com/blog/become-mysql-
dba-blog-series-configuration-tuning-performance
! Contact: krzysztof@severalnines.com
30
Thank You!