In C (and C++) you can specify that a variable should take a specific number of bits of storage by doing “uint32_t foo:4;” rather than just “uint32_t foo”. In this example, the former uses 4 bits while the latter uses 32bits. This can be useful to pack many bit fields together.

Or, that’s what they’d like you to think.

In reality, the C spec allows the compiler to do just about anything it wants with these bitfields – which usually means it’s something you didn’t expect.

For a start, in a struct -e.g. “struct foo { uint32_t foo:4; uint32_t blah; uint32_t blergh:20; }” the compiler could go and combine foo and blergh into a single uint32_t and place it somewhere… or it could not. In this case, sizeof(struct foo) isn’t defined and may vary based on compiler, platform, compiler version, phases of the moon or if you’ve washed your hands recently.

Where …

The 2014 edition of Percona Live London brought together attendees from 30 countries to hear insightful talks from leaders in the MySQL community. The conference kicked off on Monday with a full day of tutorials followed by the very popular Community Dinner featuring a double decker bus shuttle from the conference to the event.

Tuesday started with keynote talks by representatives from MySQL, VMware, HGST, Codership, and Percona. I particularly enjoyed the talks by Tomas Ulin of MySQL (which highlighted the upcoming MySQL 5.7 release) and Robert Hodges of VMware (which focused on the evolution of MySQL). The remainder of the day was filled by six time slots of breakout sessions (30 sessions in all) broken into 10 tracks. The day wrapped up with the always popular Community Networking Reception. Attesting to the …

Logging into a MySQL server is generally dead simple—supply a username, a password, and you’re all set!

There are, however, more complex use cases such as when making use of our Enterprise Authentication plugins. It’s also sometimes helpful to have a more detailed understanding of what happens “under the hood”. So I’ll attempt to lift the hood and walk you through all of the nitty-gritty details regarding exactly just what happens when you log into a MySQL server.

Firstly, there are no less than 4 “users” involved in the authentication process. And a distinction between a user id and a user account exists. And it gets more and more advanced with each release. So I thought I’d take the time to try to walk you through the MySQL authentication process and clarify once and for all the stages involved, the information exchanged at each stage and the expected outcome.

I’ll use the tools that come …

While playing with MySQL 5.7.5 on POWER8, I came across a rather interesting bug (74775 – and this is not the only one… I think I have a decent amount of auditing and patching to do now) which made me want to write a bit on memory barriers and the volatile keyword.

Memory barriers are hard.

Like, super hard. It’s the kind of thing that makes you curse hardware designers, probably because they’re not magically solving all your problems for you. Basically, as you get more CPU cores and each of them have caches, it gets more expensive to keep everything in sync. It’s quite obvious that with *ahem* an eventually consistent model, you could save a bunch of time and effort at the expense of shifting some complexity into software.

Those in the MySQL world should recognize this – we’ve been dealing with asynchronous replication for well over a decade …

A lot a great announcements were done today at the Visual Studio Connect event. And one of the things we are more excited about is hearing that there is a new edition of Visual Studio: Visual Studio 2013 Community.

Did your logging stop working after you set up logrotate? Then this post might be for you.

Archive your log files!

Some time ago, Peter Boros wrote about Rotating MySQL Slow Logs safely, explaining the steps of a “best practice” log rotate/archive. This post will add more info about the topic.

When running logrotate for MySQL (after proper setting the /etc/logrotate.d/mysql conf file) from anacron, there’s a situation that you might potentially face if the user and password used to execute the “flush logs” command is stored in, for example, /root/.my.cnf file.

The situation:

You might find out that you have a new MySQL log file ready to receive data, but nothing is being written to it.

Why did this happen?

The logrotate script is executed, but the postrotate …

Yesterday, I got the basics going for MySQL Cluster on POWER. Today, I finished up a couple more patches to improve performance and ran some benchmarks.

This is on a 3.7Ghz POWER8 machine with non-balanced memory (only 2 of the 4 NUMA nodes have memory, so we have less total memory bandwidth than we could have, plus I’m going to bind ndbmtd to the CPUs in these NUMA nodes)

With a setup of a single replica and two data nodes on the one machine (each bound to a specific NUMA node), running the flexAsync benchmark on MySQL Cluster 7.3.7, I could get around:

• 3.2 million reads/sec
• 2.6 million deletes/sec
• 2.4 million updates/sec
• 2.4 million inserts/sec.

So, that’s at least in the right ballpark for a first go.

(I’m running this on a big endian host …

Not a ground-breaking post here, but if you are interested in knowing more about the innodb_log_block_size variable, or if you use SSD cards and/or large InnoDB log files on ext4, then this is for you.

I’d read about it before briefly before, but didn’t give it too much thought until I ran across the following entry in an error log the other day:

InnoDB: Warning: innodb_log_block_size has been changed
from default value 512. (###EXPERIMENTAL### operation)

This got me wanting to know more.

Basically, this variable changes the size of transaction log records. Generally, the default of 512 is a good value. However, it has been found that setting it to 4096 has been beneficial when using SSD cards. (Note that while it is possible to set this to a value other than 512 or 4096, those are currently the only 2 values that make sense to use.)

Also, it has been found that 4096 is the best setting if you run …

So some users complete a yum install of MySQL and expect to be able to use the following  command to start the MySQL server::  /etc/init.d/mysql start only to get "No such file or directory"
So this is a quick post to help use the systemctl command.  You are likely to see this: # systemctl list-unit-files | grep mysql
mysqld.service                              disabled
First I would recommend go to tools.percona.com and create a valid my.cnf file. 
So the solution is easy, we just need to enable this so the database can start on server start up.
#systemctl enable mysqld.service
ln -s '/usr/lib/systemd/system/mysqld.service' '/etc/systemd/system/mysql.service'
ln -s …

So, I’ve written previously on MySQL on POWER, and today is a quick bit of news about MySQL Cluster on POWER – specifically MySQL Cluster 7.3.7.

I ran into three main issues in getting some flexAsync benchmark results. One of them was the fact that I wanted to do this in the middle of all the POWER8 machines I usually use moving buildings (hard to run benchmarks when computers are packed up in boxes on a truck).

The next issue was that ndbmtd (the multi-threaded data node) needs memory barriers for the magic message passing stuff between threads. So, that’s pretty easy (about an eight line patch).

The next issue was in the results from flexAsync, it turns out 32bit math is a bad idea with results from my POWER8 box.

My preliminary performance numbers are fairly promising (actually… what is the world record for a single machine and NDB these days? Single data node?). I think there’s a bit more low …