Shinguz's blog

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.

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:

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.

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...

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 ]

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:

With MySQL 5.0 the database provides trigger functionality on INSERT, REPLACE, UPDATE and DELETE.

Those of you who know some other RDBMS know, that there are also some system events where one would like to have triggers.

Unfortunately MySQL does not (yet) provide such functionality. This is sad because as database administrator this would be sometimes very helpful.

But you can build your own LOGON and STARTUP trigger.

MySQL provides some hooks for these events...

Recently the question came up if it is faster to restore a MySQL cluster when all nodes are up or only ONE node from each node group during restore.

The answer from our gurus was: All nodes up during restore! I wanted to find out why. So I set up the following cluster and started to measure:

MySQL Cluster set up

What is MyEnv?

MyEnv is a tool to run several MySQL or MariaDB database instances on ONE host. You can even run multiple database instances with different binary versions. We call this multi-instance set-ups.

With MyEnv a multi-instance set-up is more comfortable to handle than with mysqld_multi (old) or Systemd services (new) and it provides more useful functionality.

It is possible to use an active - active shared-disk cluster in MySQL in some cases. For doing this you have to fulfill the following requirements:

Transaction performance relates among other things from I/O performance. This means hard disk performance.

Hard disk performance

When you select a hard disk, an important feature to consider is the performance (speed) of the drive. Hard disks come in a wide range of performance capabilities. As is true of many things, one of the best indicators of a drive’s relative performance is its price. An old saying from the automobile-racing industry is appropriate here: "Speed costs money. How fast do you want to go?"

In a round-robin database (RRD) usually time-series data like network bandwidth, temperatures, CPU load etc. is stored. The data is stored in the way that system storage footprint remains constant over time. This avoids resource expensive purge jobs and reduces complexity.

MySQL does NOT yet provide this kind of storage engine. Although some people were thinking about and some prototypes exists.

The price for a 160 Gbyte disk will be around USD 15'000. This is still a bit expensive. But the access time is around 0.5 ms (both for reading and writing) which is around 10 times faster than a normal 15'000 rpm SCSI disk! The disk has NO cache because it is a cache itself (according to the supplier. Maybe this will change in the future). And the lifetime of a cell is > 5 mio writes. For the same performance one needs usually an array of around 10 disks. If your database is heavily write-I/O bound you should consider this solution.

MySQL provides its replication for High Availability (HA) and for read Scale-out. Generally it is known that in a MySQL replication you can only replicate from one Master to many slaves. In this paper it is shown how a set-up can look like to replicate from two masters to one slave.

Caution: Handle this information with care!!!

Probably the answer to this question is already known. But we want to prove it and by the way learn to deal with MySQL and oprofile.

Materialised View (MV) is the pre-calculated (materialised) result of a query. Unlike a simple VIEW the result of a Materialised View is stored somewhere, generally in a table. Materialised Views are used when immediate response is needed and the query where the Materialised View bases on would take to long to produce a result. Materialised Views have to be refreshed once in a while. It depends on the requirements how often a Materialised View is refreshed and how actual its content is.

A Federated Table is a table which points to a table in an other MySQL database instance (mostly on an other server). It can be seen as a view to this remote database table. Other RDBMS have similar concepts for example database links.

Read more about the pittfalls with MySQL Federated Tables...

We have added some more tools to our MySQL consulting tools collection.

We have added some more database benchmarks to our collection.

More details you can find on our Benchmarking page.

One of the big advantages of MySQL is its concept of pluggable Storage Engines (SE). This means you can choose the most optimal storage engine for your needs. This also has a disadvantage: You have to know what you are doing...

More details you can find here: MySQL pluggable Storage Engines (SE).