I recently encountered a situation in which after running Percona Toolkit’s pt-table-checksum on a customer system, 95% of the table on the MySQL master was different on the MySQL slave. Although this table was not a critical part of the infrastructure, from time to time, writes to the table from the master would break replication. Additionally, this table has about 6 million rows, and running pt-table-sync would take sometime. Rebuilding the slave from backup of the master would not be an easy option as well since the slave acts as an archive where it has a lot more data than the master.

So how did we solve it? With pt-online-schema-change and a NOOP ALTER.

pt-online-schema-change --alter 'ENGINE=INNODB' D=dbname,t=tblname

How is it …

I was fortunate to attend an Ops discussion about databases at the OpenStack Summit Atlanta this past May as one of the panelists. The discussion was about deadlock issues OpenStack operators see with Percona XtraDB Cluster (of course this is applicable to any Galera-based solution). I asked to describe what they are seeing, and as it turned out, nova and neutron uses the SELECT … FOR UPDATE SQL construct quite heavily. This is a topic I thought was worth writing about.

Write set replication in a nutshell (with oversimplification)

Any node is writable, and replication happens in write sets. A write …

There are a lot of tools that generate test data.  Many of them have complex XML scripts or GUI interfaces that let you identify characteristics about the data. For testing query performance and many other applications, however, a simple quick and dirty data generator which can be constructed at the MySQL command line is useful.

First, let’s talk about what kind of data you can easily create with MySQL function calls:

You can generate a decimal number between zero and another number using the MySQL RAND() function like the following query (here between 0 and 10000):

SELECT RAND() * 10000;

Similarly, you can generate a random integer by adding the FLOOR() function:

SELECT FLOOR(RAND() * 10000)

You can generate a random string of 32 characters using MD5():

SELECT MD5(RAND() * 10000)

You can return a random integer between 500 and 1000 with the following:

A common migration path from standalone MySQL/Percona Server to a Percona XtraDB Cluster (PXC) environment involves some measure of time where one node in the new cluster has been configured as a slave of the production master that the cluster is slated to replace. In this way, the new cluster acts as a slave of the production environment – traditional replication takes care of getting the data into the cluster, and then Galera replication handles the intra-cluster traffic. This often works without issue, although there is one case that I’ve encountered recently where special care must be taken to properly configure the stream to ensure that replication does not break. If you use multi-valued inserts with auto-increment columns, then this post is for you.

For purposes of our discussion, assume that we have a basic 3-node PXC cluster that we’ve set up …

When a write query is sent to Percona XtraDB Cluster all the nodes store the writeset on a file called gcache. By default the name of that file is galera.cache and it is stored in the MySQL datadir. This is a very important file, and as usual with the most important variables in MySQL, the default value is not good for high-loaded servers. Let’s see why it’s important and how can we calculate a correct value for the workload of our cluster.

What’s the gcache?
When a node goes out of the cluster (crash or maintenance) it obviously stops receiving changes. When you try to reconnect the node to the cluster the data will be outdated. The joiner node needs to ask a donor to send the changes happened during the downtime.

The donor …

Here on the Percona Support team we often ask customers to retrieve disk stats to monitor disk IO and to measure block devices iops and latency. There are a number of tools available to monitor IO on Linux. iostat is one of the popular tools and Percona Toolkit, which is free, contains the pt-diskstats tool for this purpose. The pt-diskstats tool is similar to iostat but it’s more interactive and contains extended information. pt-diskstats reports current disk activity and shows the statistics for the last second (which by …

Join me online next week (September 10 at 10 a.m. PDT) for my live webinar on Migrating to Percona XtraDB Cluster.  This was a popular webinar that I gave a few years ago, so I’m doing it again with updates for Percona XtraDB Cluster 5.6 (PXC) and all the latest in the Galera world.

This webinar will be really good for people interested in getting an overview of what PXC/Galera is, what it would take to adopt it for your application, and some of the differences and challenges it brings compared with a conventional MySQL Master/slave setup.  I’d highly suggest attending if you are considering Galera in your environment …

Let me start by saying a big “thank you” to the staff at Oracle for deciding to open source reducer.sh. It’s a tool I developed whilst I was working for them several years ago. Its sole purpose is to do one thing – but do it good: test-case simplification.

So, let’s say some customer just sent you 120,000 lines of SQL code and affirms that “it definitely causes a crash.” Or maybe you ran RQG (the Random Query Generator) for awhile (with the general query log turned on) and now you have a nice SQL trace which may just lead to that crash the run resulted in. Or you’re a DBA testing the company’s usual queries with Valgrind, and noticed that 2 in 1000 queries give a Valgrind warning in the mysqld error log – you’re just not sure which one. Or maybe you’re a developer, and during testing you saw that a SELECT query output did not look the way it should – the output was “7″ where it should have been “5″ – the …

Given the recent excitement & interest around OpenStack I wanted to make sure I was ready to conduct appropriate evaluations of system performance.  I generally turn to sysbench since it comes with a variety of different tests (accessed via –test= option interface), including:

• fileio – File I/O test
• cpu – CPU performance test
• memory – Memory functions speed test
• threads – Threads subsystem performance test
• mutex – Mutex performance test

As you can see, sysbench lets you stress many of the fundamental components of your hardware and infrastructure, such as your disk subsystem, along with your CPUs and memory. An additional option exists that is designed to perform synthetic stress testing of MySQL, and I was surprised when I didn’t see it in the above list on version 0.5, as it used to show up as “oltp – OLTP test”. What happened to –test=oltp …

Galera replication for MySQL brings not only the new, great features to our ecosystem, but also introduces completely new maintenance techniques. Are you concerned about adding such new complexity to your MySQL environment? Perhaps that concern is unnecessarily.

I am going to present here some simple tips that hopefully will let fresh Galera users prevent headaches when there is the need to recover part or a whole cluster in certain situations. I used Percona XtraDB Cluster (project based on Percona Server and Galera library + MySQL extensions from Codership) to prepare this post, but most if not all of the scenarios should also apply to any solution based on MySQL+Galera tandem you actually chose, whether these are binaries from Codership, MariaDB Galera Cluster or …