This post explains the inherent problem of running online schema changes in MySQL, on tables participating in a foreign key relationship. We’ll lay some ground rules and facts, sketch a simplified schema, and dive into an online schema change operation. Our discussion applies to gh-ost, pt-online-schema-change, and VReplication based migrations, or any other online schema change tool that works with a shadow/ghost table like the Facebook tools. Why Online DDL? # Online schema change tools come as workarounds to an old problem: schema migrations in MySQL were blocking, uninterruptible, aggressive in resources, replication unfriendly.
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Vitess introduces a new way to run schema migrations: non-blocking, asynchronous, scheduled online DDL. With online DDL Vitess simplifies the schema migration process by taking ownership of the operational overhead, and providing the user a simple, familiar interface: the standard ALTER TABLE statement. Let’s first give some background and explain why schema migrations are such an issue in the databases world, and then dive into implementation details The relational model and the operational overhead # The relational model is one of the longest surviving models in the software world, introduced decades ago and widely used until today.
I just posted an article on the Percona Community Blog. You can access it following this link:
A Nice Feature in MariaDB 10.3: no InnoDB Buffer Pool in Core Dumps
I do not know if I will stop publishing posts on my personal blog or use both, I will see how things go. In the rest of this post, I will share why I published there and how things went in the process.
So there is a Percona
MariaDB 10.3 is now generally available (10.3.7 was released GA on 2018-05-25). The article What’s New in MariaDB Server 10.3 by the MariaDB Corporation lists three key improvements in 10.3: temporal data processing, Oracle compatibility features, and purpose-built storage engines. Even if I am excited about MyRocks and curious on Spider, I am also very interested in less flashy but still very important changes that make running the database in production easier. This post describes such improvement: no InnoDB Buffer Pool in core dumps.
Hidden in the …[Read more]
In this blog, I’m going to investigate why the error below has been appearing in a special environment I’ve been working with on the last few days:
Last_IO_Errno: 1236 Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'The slave is connecting using CHANGE MASTER TO MASTER_AUTO_POSITION = 1, but the master has purged binary logs containing GTIDs that the slave requires.'
The error provides the right message, and explains what is going on. But sometimes, it can be a bit tricky to solve this issue: you need additional information discovered after some tests and readings. We try and keep Managed Services scripted, in the sense that our advice and best …[Read more]
The pt-online-schema-change tool has been a workhorse for years, allowing MySQL DBAs to alter tables with minimal impact to application workload, and before MySQL had native online alter capabilities. Although consistently grateful for the tool’s existence, I never liked the messiness of having to use and clean up triggers, and most DBAs have a horror story or two to tell about using any of the online alter methods.
When Github’s online schema transmogrifer (gh-ost; https://github.com/github/gh-ost) was released, then, I jumped on the chance to test it out.
Following are my testing notes. I tried out the flags available to run gh-ost from different locations in a replication setup, both functionally and under load. The load applied was via mysqlslap, using a …[Read more]
In the Install and Configure HOWTO, we learned how to install and configure a MaxScale Binlog Server. In this HOWTO, I will present the common operations that you might need to perform when using this software. Those operations include:
Purging Binary Logs, Chaining Binlog Servers, Saving Binary Log Files in the Non-Default Directory, Downloading Binary Logs other than First, Listing Connected
On a recent consulting engagement, the PSCE team were charged with what can be considered a fairly common task of synchronising tables between master and slave in MySQL Replication. On this occasion the schema contained both foreign key constraints and triggers, this post describes how we avoided the potential problems related to such an operation.
The process to synchronise tables in MySQL is to first identify the differences between tables and then execute queries which bring those tables into a consistent state. The first part of the process can be handled by the pt-table-checksum tool, which steps through the table analysing sets of rows (chunks) and recording a checksum value. Then taking advantage of replication, the same process occurs on each of the slaves and the checksums can then be compared. Once the entire table has been processed, a second tool pt-table-sync can be used …[Read more]
High season is coming, how do you make sure that MySQL will handle the increased load? Stress tests could help with that, but it’s not a good idea to run them in a production environment. In this case Select_scan, Select_full_join and other MySQL counters could quickly give you an idea of how many queries are not performing well and could cause a performance degradation as the load goes up.
Select_scan from SHOW GLOBAL STATUS indicates how many full table scans were done since last MySQL restart. Scanning the entire table is a resource intensive operation. It also forces MySQL to store unnecessary data in the buffer pool, wasting memory and IO resources.
Full scan of a tiny table would be quite fast so missing indexes could stay invisible until the load rises or the dataset grows up. This could also be the case for developers who work with too small data sets on their dev boxes. To prevent performance issues all newly added …[Read more]
When you're testing out a new version of MySQL in a
non-production environment there is a temptation to go wild and
turn on all kinds of new features. Especially if you're
reading the changelogs or the manual and scanning through
options. You want to start with the most reasonable set of
defaults, right? Maybe you're even doing benchmarks to
optimize performance using all the new bells and whistles.
Resist the temptation! If your goal is to upgrade your production environment then what you really want is to isolate changes. You want to preform the upgrade with as little to no impact as possible. Then you can start turning on features or making changes one-by-one.
Why? Anytime you're doing a major upgrade to something as fundamental as your core RDBMS, there are many ways things can go wrong. Performance regressions & incompatible changes, client/server incompatibilities …
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