Tag Archives: gotchas

Regular viewers will know that I don’t think favourably of MySQL.
Here is yet another reason. Let’s create an InnoDB table:

mysql> CREATE TABLE `User_` (
mysql> ...
mysql> ) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Query OK, 0 rows affected, 1 warning (0.04 sec) 

One warning, but we’re running this as part of an import, so we’ll
fail to spot this and even if we did, we wouldn’t be able to get it back
out of mysql because SHOW WARNINGS only shows the last command.
So let’s look at the table we just created:

mysql> show create table User_G
*************************** 1. row ***************************
       Table: User_
Create Table: CREATE TABLE `User_` (
...
) ENGINE=MyISAM DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

Eh? what’s going on? We asked for InnoDB, but have got a MyISAM
table. Lets look at the engines available.

mysql> show engines;
+------------+----------+----------------------------------------------------------------+
| Engine     | Support  | Comment                                                        |
+------------+----------+----------------------------------------------------------------+
| MyISAM     | DEFAULT  | Default engine as of MySQL 3.23 with great performance         |
| MEMORY     | YES      | Hash based, stored in memory, useful for temporary tables      |
| InnoDB     | DISABLED | Supports transactions, row-level locking, and foreign keys     |
| BerkeleyDB | NO       | Supports transactions and page-level locking                   |
| BLACKHOLE  | NO       | /dev/null storage engine (anything you write to it disappears) |
| EXAMPLE    | NO       | Example storage engine                                         |
| ARCHIVE    | YES      | Archive storage engine                                         |
| CSV        | YES      | CSV storage engine                                             |
| ndbcluster | DISABLED | Clustered, fault-tolerant, memory-based tables                 |
| FEDERATED  | YES      | Federated MySQL storage engine                                 |
| MRG_MYISAM | YES      | Collection of identical MyISAM tables                          |
| ISAM       | NO       | Obsolete storage engine                                        |
+------------+----------+----------------------------------------------------------------+
12 rows in set (0.00 sec)

Oh, so innodb has been disabled. We can fix that easily by removing
skip-innodb from my.cnf.

root@cmsdb01:/var/log# grep skip-innodb /etc/mysql/my.cnf
root@cmsdb01:/var/log#

But hang on a second, that’s not in the config file. What’s going on?
It turns out that the reason InnoDB is disabled is because of the
innodb_log_file_size setting not matching the files on disk.

root@cmsdb01:/var/log# grep innodb_log_file_size /etc/mysql/my.cnf
innodb_log_file_size            = 512M
root@cmsdb01:/var/log# ls -lh /var/lib/mysql/ib_logfile*
-rw-rw---- 1 mysql mysql 5.0M 2006-12-19 18:39 /var/lib/mysql/ib_logfile0
-rw-rw---- 1 mysql mysql 5.0M 2006-12-19 18:39 /var/lib/mysql/ib_logfile1

Rumour has it that you can just stop MySQL, delete these log files
and start MySQL again. I’m yet to try this as the server in question is
in production use. The alternative is to change the
innodb_log_file_size setting to match the file.

So in summary the problems with MySQL are:

• Not logging warnings anywhere useful.
• Converting engine types with a warning rather than throwing an
  error. This can be fixed by setting sql_mode to include
  NO_ENGINE_SUBSTITUTION.
• Starting up and disabling InnoDB when there is a problem rather than
  failing to start, giving a false impression that everything is
  working.

MySQL has not impressed me this week.