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Copyright 2015 Severalnines AB
Schema Changes for MySQL Replication & Galera Cluster
August 25, 2015
Krzysztof Książek
Severalnines
krzysztof@severalnines.com
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! 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
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“Become a MySQL DBA” series

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! 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
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Agenda

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Schema changes in MySQL
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! Application development
! New features to existing
modules
! New modules
! Data management and
archiving (partitions)
! Schema optimization
! Performance optimization
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Why schemas need changes?

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! 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
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Types of schema changes

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! 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
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Schema changes - limitations

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! 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
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Schema changes - limitations

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! 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
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Schema changes - limitations

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Ways to perform a schema change
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! Online DDL
! Rolling schema upgrade
! Online schema change
! Rolling Schema Upgrade for Galera
! Total Order Isolation for Galera
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Methods to perform a schema change

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! 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
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Online DDL

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! 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
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Online DDL

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! 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
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Online DDL

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! 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
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Rolling Schema Upgrade

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! 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
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Online Schema Change - external tools

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Online Schema Change - external tools
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! 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
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pt-online-schema-change

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! 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
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pt-online-schema-change

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Schema changes in Galera Cluster
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! 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
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Total Order Isolation

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! 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
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Rolling Schema Upgrade

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Rolling Schema Upgrade
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Different scenarios for a schema
change
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! 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
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Small tables (up to 10s of a direct DDL)

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! 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.)
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Medium tables (20 - 30m, up to 1h)

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! 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)
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Large tables (more than 1h, up to 12h)

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! 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
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Very large tables (more than 12h)

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! 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
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What to keep in mind?

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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
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Thank You!