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Logging users to the MySQL error log

Problem

A customer recently showed up with the following problem:

With your guidelines [ 1 ] I am now able to send the MySQL error log to the syslog and in particular to an external log server.
But I cannot see which user connects to the database in the error log.

How can I achieve this?

Idea

During night when I slept my brain worked independently on this problem and in the morning he had prepared a possible solution for it.

Can you trust your MySQL backup?

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Today a customer with corrupted data files showed up. When we enquired a bit more he told us that he had a broken I/O controller. This is one of the worst things which can happen to you!

The reason is the following: When a I/O controller starts to die it often does not happen immediately. The controller dies slowly producing more and more corrupt data. When you just write data without checking or reading them it can take days or even weeks until you discover the problem.

What is CHECK TABLE doing with InnoDB tables?

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Recently we had a case where a customer got some corrupted blocks in his InnoDB tables. His largest tables where quite big, about 30 to 100 Gbyte. Why he got this corrupted blocks we did not find out yet (disk broken?).

When you have corrupted blocks in InnoDB, it is mandatory to get rid of them again. Otherwise your database can crash suddenly.

MySQL on VMware Workstation/DRBD vs. VMWare ESX Server/SAN

Or an active-active fail-over cluster à la VMware.

Today I have learned about a totally crazy/cool looking architecture where the expensive VMware ESX server was replace by a free/cheap VMware Workstation version in combination with DRBD.

Basically DRBD we name the poor man's SAN and that is exactly what this customer is doing. He replaced the SAN with DRBD and now he can easily move one VMware instance to the other host. Possibly it is not that flexible and powerful as an ESX Server but also not so expensive...

The architecture looks as follows:

The battle against Oracle is probably over but has the real war begun yet?

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According to different sources from the web the decision about the Oracle - Sun merger has been approved by the European commission soon. So at least in the West it is clear what is going on. Let us see what the East decides... [ 1 ], [ 2 ].

MySQL reporting to syslog

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There are 2 different possible situations you can face when you have to deal with MySQL and syslog:

  • MySQL is used as back-end for syslog to store the logging information. [ 6 ]
  • MySQL itself should report to the syslog.

In this blog article we look at the second situation: How can you make MySQL reporting to the syslog.

My wish for the New Year: MySQL DBA's, please install iostat on your servers!

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iostat is a very handy tool to help you investigating what kind of performance problems you have. Especially your databases can cause a lot of troubles to your I/O system and thus it would be very nice if every DBA has installed iostat on all of his MySQL database servers.

MySQL useful add-on collection using UDF

I really like this new toy (for me) called UDF. So I try to provide some more, hopefully useful, functionality.

The newest extension I like is the possibility to write to the MySQL error log through the application. Oracle can do that since long. Now we can do this as well...

A list of what I have done up to now you can find here:

Using MySQL User-Defined Functions (UDF) to get MySQL internal informations

In one of my previous posts I was writing about how to read other processes memory [ 1 ]. As an example I tried to get the value of the hard coded MySQL internal InnoDB variable spin_wait_delay (srv_spin_wait_delay).

In this example we were using gdb or the operating system ptrace function to retrieve this value. This method has the disadvantage that it is pretty invasive.

Determine in MySQL if we are in summer time or winter time (daylight saving time, DST)

Recently a colleague at Sun was asking me if MySQL can tell him to determine if we are currently in summer time or winter time. He was doing some data analysis of his house where he has installed solar panels.

I am not aware of what he wants to do exactly, but possibly he wants all the data in solar time. So UTC could help him because UTC does not change much over time.

Next thing which came to my mind is, that possibly the good place to do such mathematical calculations is the application code and not the database.

Reading other processes memory

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As you probably have experienced yet MySQL does not always provide all internal information as you might want to have them and as you are used to have from other RDBMS.

MySQL plans to improve this implementing the/a performance schema and its probably already partly done in MySQL 5.4. But who knows when this will be finished and what it contains at all...

MySQL licenses for dummies

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The following summary shows my personal understanding of MySQL 5.1 licenses, packages and products. It does not necessarily reflect 100% the way MySQL understands it. But after all the discussions I hope it is as close as possible to the reality:

MySQL Embedded Database Server

(Download -> OEM Software)

Why does MySQL Cluster takes so long for --initial?

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This week we had a very interesting problem at a customer: They complained, that their MySQL Cluster takes about 1 hour for a --initial start-up. After some tuning on the hardware and the config.ini they brought it down to around 40 minutes. But this is still unacceptable long...

This sounds a little strange to me. But let us have a look at their config.ini first. It looked more or less like the following (which is already tuned!):

MySQL Multi-Master – Single-Slave – Replication (Episode 2)

Introduction

One of the features that make MySQL so great is its easy replication set-up. If you are experienced and know-how to do it, it takes you about 15 minutes to set-up a slave. What you have in the end is a replication from one master to one or several slaves. So you can build a top-down data stream pyramid and spread your data on many slaves.

MySQL Replication

Test application for MySQL high availability (HA) set-up

When I set-up a MySQL HA environment for customers I usually do some final fail over tests after configuring the whole beast.

To check if the application behaves like expected I always run my little test application (test.sh) from the server(s) where the customers application runs. It displays "graphically" how the application behaves and you can show to the customer immediately what is going on...

Make sure, that you point it to the virtual IP (VIP) or the load balancer (LB).

Citation of the week

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Das dreieckige Rad hat gegenüber dem viereckigen einen gewaltigen Vorteil: Ein Rumms weniger pro Umdrehung!

Translation:

The triangular wheel has one enormous advantage over the quadrangular: One knock less per revolution!

Maybe not new, but I have not heard it yet and I love it. It was about reinventing functionality in a well known product...

MySQL Cluster: No more room in index file

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Recently we were migrating an InnoDB/MyISAM schema to NDB. I was too lazy to calculate all the needed MySQL Cluster parameters (for example with ndb_size.pl) and just took my default config.ini template.

Because I am really lazy I have a little script doing this for me (alter_engine.pl).

But suddenly my euphoria was stopped abruptly by the following error:

Possible memory leak in NDB-API applications?

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A customer has recently experienced a possible memory leak in its NDB-API application. What he did was something like:

# ps aux | grep <pid>

over time and then he saw the RSS increasing. When he would have had a look a little longer he would have seen that the RSS consumption would increase up to a certain level and then becomes stable. Which is the expected behavior.

Possible Memory Leak

Active/active fail over cluster with MySQL Replication

Electing a slave as new master and aligning the other slaves to the new master

In a simple MySQL Replication set-up you have high-availability (HA) on the read side (r). But for the master which covers all the writes (w) and the time critical read (rt) there is no HA implemented. For some situations this can be OK. For example if you have rarely writes or if you can wait until a new Master is set up.

But in other cases you need a fast fail-over to a new master.

Typical automated MySQL maintenance jobs, query cache

The following maintenance jobs are typically run against a MySQL database:

  • Backup
  • Clean-up binary logs
  • Optimize tables
  • Purge query cache
  • Rotate binary logs

Backup

A backup is not a typical maintenance job. But it behaves more or less like one. The backup should be done regularly depending on the restore/PITR (Point in Time Recovery) requirements.

My thoughts about MySQL (Cluster) replication

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According to Johans wishes I write down my concerns about MySQL (Cluster) replication. These items are things I run again and again into it with customers:

Some more details about DiskSyncSize

The parameter DiskSyncSize is a MySQL Cluster parameter and was added in MySQL 5.1.23.

After the amount of stored bytes of data per file, the data node will fsync (flush) the LCP file to disk, even if a fsync is not needed for consistency.

This is done because the OS will otherwise buffer all the writes, and when a fsync is really needed, it can take a lot of time...

Originally this parameter was hard coded. Now it defaults to 4 Mbyte.

With MySQL-Enterprise Montior through firewalls

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Sometimes it is nice to show customers the functionality of MySQL-Enterprise Monitor (aka Merlin). I install the agents on the servers and the dashboard runs on my laptop. But very often only ssh is open to these servers.

So how to dig a whole through the firewall for MySQL-Enterprise Monitor?

# ssh -R 18080:localhost:18080 oli@where_the_agent_sits

Maybe trivial for you but for me its hard to remember...

Sparse files

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What is a sparse file?

"A sparse file is a file where space has been allocated but not actually filled with data. These space is not written to the file system. Instead, brief information about these empty regions is stored, which takes up much less disk space. These regions are only written to disk at their actual size when data is written to them. The file system transparently converts reads from empty sections into blocks filled with zero bytes at runtime." [ 1 ]

Ranking in MySQL results

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A friend of me asked me long time ago: "How can I have a ranking on a result with MySQL?". Now I found some time to write it down:

Lets do first some preparation for the example:

CREATE TABLE sales (
  id     INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
, fruit  VARCHAR(32)
, amount DECIMAL
);

INSERT INTO sales
VALUES (NULL, 'apple', 12.75), (NULL, 'orange', 1.89), (NULL, 'pear', 19.23)
     , (NULL, 'banana', 4.25), (NULL, 'cherry', 123.75), (NULL, 'plum', 23.15)
;

Now lets query:

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