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When a MySQL table was last touched

In our last customer project we had around 600 Gbyte of data in a MySQL database. Because this database consumed a significant amount of our disk space and backups with the InnoDB backup tool took pretty long we wanted to find out if we could get rid of some of the tables.

This application was growing over the last 10 years and it was not clear if some tables are still in use or not.

But how to find out when a table was touched last? MySQL/InnoDB theoretically could know about but does not report this information.

Dumping BLOB's from the MySQL database

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A customer who is handling digital certificates had a problem with one of those. So we had to investigate.

Because the certificate is in binary form it is stored in a BLOB and we had to extract it from the database to do some verifications.

What first came to my mind was to extract the certificate with the SELECT INTO OUTFILE command. But the verification tool complained and told us that the certificate has a wrong format.

Configuration of MySQL for Shared Hosting

If you ask around about shared hosting setups with MySQL everybody is frightened. In fact it looks like shared hosting is one of the most difficult setups you can get.

The number of users is big, the number of tables huge and the load pattern is completely unpredictable and the queries often very, let us say: non-optimal.

Here one of the DBA wisdoms come into play: Controlling developers is like herding cats.

If you talk to the Shared MySQL Hoster they confirm that this setups are very demanding!

MySQL HA (high availability) Cluster cookbook

In the following article I have summarized some steps and hints to set-up a MySQL active/passive fail-over Cluster also sometimes called MySQL HA.

With such a set-up you can achieve a 99.99% (4x9) HA set-up for MySQL (52 minutes downtime per year). The same procedure also works for PostgreSQL, Oracle and other database systems running on Linux (DRBD runs on Linux only).

The concept

The concept of an active/passive fail-over Cluster is the following:

MySQL Performance Monitor with DRBD monitoring capabilities

The FromDual Performance Monitor for MySQL has now a new module for monitoring DRBD. Since more an more of our customers want to run MySQL High Availability solutions based on DRBD we have implemented this module.

We gather all information which are reported with the command:

Be careful when using SAN

Be careful when using SAN (Storage Area Networks) or similar shard storage solutions (and any other virtualization, consolidation or cloud solutions).

This week it happened again: A customer called us because he had some troubles with his on-line shop (note the date!). Everybody in his company complained that the databases are answering slowly.

MySQL Cluster - Cluster circular replication with 2 replication channels

A few days ago I had to deal with MySQL Cluster replication. I did not do this for a while so I was prepared to expect some interesting surprises once again.

For those who MySQL Cluster - Cluster circular replication is the daily business they may skip this article. For all the others they possibly can profit from our learnings.

I am talking about the following MySQL Cluster set-up:

How can I find what InnoDB version I am using?

In the old days everything was simpler. We had one maker of our favourite database management system and possibly the choice between different Storage Engines. Mostly the decision has to be taken between MyISAM and InnoDB. When you care about your data integrity you have chosen InnoDB.

Nowadays it is more complex. We have several different makers of our favourite database management system: Oracle/MySQL, Monty Program AB and Percona with their products: MySQL, MariaDB and Percona Server.

Impact of indices on MySQL

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It is generally well known that indexes help a lot to speed up database queries (especially SELECT but also UPDATE and DELETE). It is less known, that indexes also have some disadvantages.

One of these disadvantages is, that indexes require space in memory and on disk. An other disadvantage of indexes is, that they slow down DML statements like INSERT and DELETE.

We often see at our customers that they do not realized this behavior. Now I found the time to show the impact of (too) many indexes graphically.

MySQL logon and logoff trigger for auditing

A while ago I did some research about MySQL audit functionality and logon a and logoff triggers. MySQL and MariaDB provide a logon trigger in the form of the init_connect variable but no logoff trigger where most of the work for auditing would be done. When we would have a logoff trigger we could track the login and possibility some activity of a user and implement auditing functionality.

MyEnv for MySQL Multi-Database set-ups

This week I showed one of my customers our MyEnv. He was very interested in it and suggested to make it known in public. In fact MyEnv is available for download already several years...

But I did not have the heart yet to announce it more publicly because it was not end user ready at all. So I used the weekend to make it nicer, consolidated some of the code, dropped old stuff etc. Now I think it is acceptable to use for public but not perfect.

How many warm MyISAM key blocks do you have?

When you are working with MyISAM [ 1 ] tables MySQL provides a feature called the Midpoint Insertion Strategy [ 2 ]. You can enable it with the parameter key_cache_division_limit [ 3 ].

By default, the key cache management system uses a simple LRU [ 4 ] strategy for choosing key cache blocks to be purged:

MyISAM key buffer

Disadvantages of explicitly NOT using InnoDB Primary Keys?

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We recently had the case with one of our customers where we got externally generated random hash values (up to 70 bytes) and they were used as Primary Keys in InnoDB.

As we know, this is not a very good idea because the size of all secondary indexes becomes large and because a random hash value as a Primary Key gives us a bad locality of our rows in the table [ 1 ].

Which table is hit by an InnoDB page corruption?

InnoDB is known to have crash-recovery capabilities and thus is called a crash safe storage engine (in contrary to MyISAM). Never the less under certain circumstances it seems like InnoDB pages can get corrupt during a crash and then a manual crash-recovery is needed.

MySQL Cluster Local Checkpoint (LCP) and Global Checkpoint (GCP)

MySQL Cluster is mainly an in-memory database. Nevertheless it requires a good I/O system for writing various different information to disk.

The information MySQL Cluster writes to disk are the:

  • Global Checkpoints (GCP) which are the transactions.
  • Local Checkpoints (LCP) which is a dirty image of the data.
  • Backup.

In the following schema (a 2-node Cluster) you can see what is related to each other:

lcp_gcp.png

How the MySQL Optimizer with MySQL Cluster is cheating you...

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At a customer we had a nice example of how the MySQL Optimizer is cheating when used in combination with the MySQL Cluster. The customer had queries running not too slow in the development environment but when he tried them on the acceptance test environment (with more data) the query was running much too long which was unacceptable because this query can occur many times per second.

What has happened?

First of all we had a look at the execution plan of the query generated by the MySQL Optimizer:

FromDual - The MySQL consulting company goes operational today!

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Hello everybody,

One month earlier than planned we have the great pleasure to announce you that the company called FromDual goes operational today!

We are excited about this step and it is an new era in our personal evolution to get back in full-contact with customers and solve their real life day-to-day MySQL problems.

So we are happy hearing from you and to help you solving your individual MySQL problems...

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?

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?

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

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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.

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