The other day we began to encounter weird and random errors on small and innocent queries that shouldn’t give any errors at all. It all lead to one of our most basic health checks failing for no apparent reason.

The first clue that popped into our minds was related to case-sensitivity, since the failing check was looking for the column names of the only table in the schema with UPPER CASE name. This symptom was especially weird since all of this was happening on MySQL setups running on Windows, and so we’re not sure if this was being caused by some internal code library, MySQL, or Windows itself.

Since that was the only clue we then had, it seemed obvious that we should start fiddling with the lower_case_table_names system variable. After a couple of restarts, this approach was leading us nowhere, so we finally gave up on it.

Then, we had the brilliant idea of actually executing that statement by hand on the command line to see what happened:

C:\pythian>mysql -uXXXX -p XXXX -e "desc TABLE_NAME"
Enter password: *****
ERROR 1 (HY000) at line 1: Can't create/write to file 'C:\MySQL      mp\#sql_634_0.MYI' (Errcode: 22)

C:\pythian>perror 22
OS error code  22:  Invalid argument

C:\pythian>

None of us remembered seeing this one before, so we stared at the monitor for a few moments, not realizing the meaning of this message. I guess our focus on the case-sensitivity was driving us away a from the real cause. So . . .  time to check the my.ini file. (more…)

Recently I had an interesting issue crop up. Due to an unfortunate migration incident in which involved master/master replication and not checking to see if replication was caught up, we ended up with an infinite replication loop of a number of SQL statements. awk helped immensely in the aftermath cleanup.

The basics of the replication infinite loop were (more…)

The 118^th edition of Log Buffer, the weekly review of database blogs, has been published on Ward Pond’s SQL Server blog.

Log Buffer is the only platform-neutral, distributed, human-edited article on database blogs. It receives several thousand views each week, and publishing an edition on your own blog brings those views to you. Hosting an edition of LB also introduces you and your blog to your colleagues in the DB blogosphere. Write me an email and I’ll get you started.

Now, here’s Ward Pond’s Log Buffer #118.

In many parts of the world times are uncertain. I live in the United States and we are in the middle of a financial meltdown that many fear may be as bad as the Great Depression. Because the world’s economies are so linked it is causing severe distress in many other countries as well. I just read that two trillion dollars have been lost from nest eggs in the last 15 months here in the States.

I am not going to turn this into a rant about who is right, who is wrong, or  what should be done about it to resolve the problem. This isn’t the place. I probably don’t even have the right answer. I have a different angle.

If you are involved with MySQL as a database administrator, or if you work directly with MySQL in some other aspect, you can probably breathe a little easier. Why is this? MySQL Server has grown in market penetration for a long time. It is now a significant section of the RDBMS pie.  I predict that this market penetration will only continue to grow. As this economic downturn/recession/whatever continues, companies will look harder for ways to save money. What better way to do so than replace your proprietary RDBMS that can cost you significant amounts of money, with MySQL Server? For all intents, the same functionality is there, the speed and flexibility is certainly there, and there is a giant company behind MySQL now, providing “enterprise-ready” support.

The market is crying right now for MySQL database administrators. We don’t cost any more than Oracle or Microsoft DBAs, you know. Just a couple of years ago, very few companies hired MySQL DBAs. They hired developers who also did database administration, or a system administrators who also managed the MySQL server. Now, as the number of database servers increases and the amount of data grows they want real, honest-to-goodness database administrators. If you have production experience with MySQL server in any significant amount you will not have any problems finding a job. I don’t think this is going to change anytime soon. So, even if your company succumbs to the times, there are others out there who need your experience. Don’t be dismayed! (more…)

This is a post about SYSDATE() and NOW() and CURRENT_TIMESTAMP() functions in MySQL.

Firstly, note is that of these three, only CURRENT_TIMESTAMP() is part of the SQL Standard. NOW() happens to be an alias for CURRENT_TIMESTAMP() in MySQL.

Secondly, note that replication does not work well with non-deterministic functions. And “hey, what time is it?” is non-deterministic. Ask it twice, with a second apart between asking, and both times you get different results (with at least second precision).

You can start to see the problem here….but there’s more…. (more…)

Quite often we need to perform a so-called “MySQL instance audit”. This common DBA procedure should give you a general view of the MySQL environment. You may be interested in a basic understanding of what kind of operation MySQL performs, how much memory does it use, or how well does it look from the performance point of view. There is no easy out-of-the-box way to do such an audit on a MySQL server. You can use SHOW STATUS and check the list of system variables, but this way can hardly be called DBA-friendly.

Fortunately there are several tools to make this process easier. Among most popular are mysqlreport and MySQLTuner. In this post I’d like to give a brief overview of MySQLTuner.

So, what can MySQLTuner do? Quoting the documentation: “MySQLTuner is a script written in Perl that will assist you with your MySQL configuration and make recommendations for increased performance and stability. Within seconds, it will display statistics about your MySQL installation and the areas where it can be improved.”

It’s not magic — they don’t use any “hidden” or unknown MySQL features to provide the report. What they do is use SHOW STATUS metrics and provide a user-friendly report, interpreting data this or that way. What makes such tools really valuable is the way they interpret that data.

(more…)

Do you have a master-slave MySQL set up?  Ever do DDL changes on the master?  You may be hit with a serious data integrity bug.  Read on.

One of our clients does a regular rename tables on the master to keep the current table small and archive off old data.  We’d occasionally be hit by a ‘duplicate key’ error on the slave on the current table and have to resolve it manually.  Digging into the issue, I managed to replicate it on demand and filed bug 39675 with MySQL, which subsequently has been verified and slated for fix, though from what it seems only in version 6.0.  The bug affects all versions of MySQL from 4.1 to 6.0.

In a nutshell, here is what happens. The rename tables command only checks for pending transactions or locks in the current session.  If there is a pending transaction in another session on the table being renamed, the rename will succeed, but the order in which the transaction is written to the binlog will be different from the order in which the transactions were applied on the master.  This means that the data on the slave will now be out of sync for this table.

Here’s a test-case:

(more…)

Yesterday evening, a friend of mine had some issues with installing DBD::mysql, and asked if I had encountered the same issue. The problem, as the output from make test showed, was that certain symbols was missing:

#     Tried to use 'DBD::mysql'.
#     Error:  Can't load '/Users/westerlund/src/perl/DBD-mysql-4.008/blib/arch/auto/DBD/mysql/mysql.bundle' for module DBD::mysql: dlopen(/Users/westerlund/src/perl/DBD-mysql-4.008/blib/arch/auto/DBD/mysql/mysql.bundle, 2): Symbol not found: _is_prefix

Fair enough, this is related to a 64-bit issue with MySQL—at least with my Perl version, which is now:

Summary of my perl5 (revision 5 version 10 subversion 0) configuration:
  Platform:
    osname=darwin, osvers=9.5.0, archname=darwin-thread-multi-64int-2level

If you try to link to a x86_64 version of MySQL, then you get the above mentioned error. So, I downloaded an x86 version of MySQL and tried again. The output from perl Makefile.PL:

  cflags        (mysql_config) = -I/Users/westerlund/src/perl/mysql-5.1.28-rc-osx10.5-x86/include  -g -Os -arch i386 -fno-common   -D_P1003_1B_VISIBLE -DSIGNAL_WITH_VIO_CLOSE -DSIGNALS_DONT_BREAK_READ -DIGNORE_SIGHUP_SIGQUIT  -DDONT_DECLARE_CXA_PURE_VIRTUAL
  embedded      (mysql_config) =
  libs          (mysql_config) = -L/Users/westerlund/src/perl/mysql-5.1.28-rc-osx10.5-x86/lib -lmysqlclient -lz -lm     -lmygcc
  mysql_config  (Users choice) = /Users/westerlund/src/perl/mysql-5.1.28-rc-osx10.5-x86/bin/mysql_config

That looks all good and nice, the right paths, versions and everything. So I compiled it OK, but when running make test again: (more…)

Martin Brown’s blog shows a pretty good way of navigating the MySQL Reference Manual. It’s worth noting, however, that finding the different topics has been a lot easier since mysql.com started using a Google appliance for its search.

I use the documentation all the time and have been doing so for years (I won’t claim that I can remember +2000 pages worth of ever-changing content). A few years back, I stopped using the search box on dev.mysql.com because the result sets were enormous, with lots of unrelated references. My technique was to do a Google site search:

For replication use the expression: replication site:http://dev.mysql.com/doc/refman/5.0/en/index.html

The result set was smaller and I would find what I was looking for relatively easily, usually within the first page.

Since the documentation team implemented the Alphabetical Index, it has succeeded the Google search as my favorite way to get the information I needed. Things are easy to find and never more than a couple of URLs away.

(more…)

Welcome to the 117^th edition of Log Buffer, the weekly review of database blogs.

For those of you who don’t know me, my name is Nicklas Westerlund, and I’m a MySQL DBA with The Pythian Group. This is my first time writing Log Buffer, and I hope I’ll do it right.

Let’s start off with SQL Server, where Simon Sabin asks if you know what concurrency is and how to improve it. And on SatisticsIO, Jason Massie focuses on the SQL Server 2008 experience instead, which should provide more inside knowledge of the technology used.

Continuing on with SQL Server 2008, the engineering team is sending loads of engineers to the SQL PASS Conference, as the SQL Server Customer Advisory Team tells us in their post on what, in their opinion, just may be the best PASS Conference yet. And if you’re into meeting engineers, then perhaps you’d also like to know how that patching is done in SQL Server 2008, which PSS SQL informs us about.

The folks over at sqlserver-qa.net also give us an overview of the SQL Server Web Edition.

Let’s move over to Oracle, where there’s still a lot of buzz about Exadata, and let’s start with with the second part of the Exadata FAQ by Kevin Closson. In that post he also mentions his interview on the Exadata with Paul and Christo here at Pythian.

(more…)