How to Optimize MySQL Tables and Defragment to Recover Space

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If your application is performing lot of deletes and updates on MySQL database, then there is a high possibility that your MySQL data files are fragmented.

This will result in lot of unused space, and also might affect performance.

So, it is highly recommended that you defrag your MySQL tables on an ongoing basis.

This tutorial explains how to optimize MySQL to defrag tables and reclaim unused space.

1. Identify Tables for Optimization

The first step is to identify whether you have fragmentation on your MySQL database.

Connect to your MySQL database, and execute the following query, which will display how much unused space are available in every table.

mysql> use thegeekstuff;

mysql> select table_name,
round(data_length/1024/1024) as data_length_mb, 
round(data_free/1024/1024) as data_free_mb 
 from information_schema.tables 
 where round(data_free/1024/1024) > 500 
 order by data_free_mb;

| table_name | data_length_mb | data_free_mb |
| BENEFITS   |           7743 |         4775 |
| DEPARTMENT |          14295 |        13315 |
| EMPLOYEE   |          21633 |        19834 |

In the above output:

  • This will display list of all tables that has minimum of 500MB of unused space. As we see above, in this example, there are 3 tables that has more than 500MB of unused space.
  • data_length_mb column displays the total table size in MB. For example, EMPLOYEE table size is around 21GB.
  • data_free_mb column displays the total unused space in that particular table. For example, EMPLOYEE table has around 19GB of unused space in it.
  • All these three tables (EMPLOYEE, DEPARTMENT AND BENEFITS) are heavily fragmented and it needs to be optimized to reclaim the unused space.

From the filesystem level, you can see the size of the individual table files as shown below.

The file size will be the same as what you see under “data_length_mb” column in the above output.

# ls -lh /var/lib/mysql/thegeekstuff/
-rw-rw----. 1 mysql mysql  7.6G Apr 23 10:55 BENEFITS.MYD
-rw-rw----. 1 mysql mysql   14G Apr 23 12:53 DEPARTMENT.MYD
-rw-rw----. 1 mysql mysql   22G Apr 23 12:03 EMPLOYEE.MYD

In this example, the EMPLOYEE.MYD file is taking up around 22GB at the filesystem level, but it has lot of unused space in it. If we optimize this table, the size of this file should go down dramatically.

2. Defrag using OPTIMIZE TABLE command

There are two ways to optimize a table.

The first method is to use Optimize table command as shown below.

The following example will optimize EMPLOYEE table.

mysql> use thegeekstuff;


You can also optimize multiple tables in a single command as shown below.


Few points to keep in mind about optimize table:

  • Optimize table can be performed for InnoDB engine, or MyISAM engine, or ARCHIVE tables.
  • For MyISAM tables, it will analyze the table, it will defragment the corresponding MySQL datafile, and reclaim the unused space.
  • For InnoDB tables, optimize table will simply perform an alter table to reclaim the space.
  • If you have indexes, it will also resort the index pages, and update the statistics.

During optimization, MySQL will create a temporary table for the table, and after the optimization it will delete the original table, and rename this temporary table to the original table.

In the above optimization, the EMPLOYEE table is an MyISAM table.

For this example, before the optimization, you’ll see the following .MYD file for the table.

# ls -lh /var/lib/mysql/thegeekstuff/EMPLOYEE.*
-rw-rw----. 1 mysql mysql   22G Apr 23 12:03 EMPLOYEE.MYD

When the “OPTIMIZE TABLE” command is running, you can see that it has created a temporary file for this table with extension .TMD. The size of this temporary file will keep growing until the optimize table is running.

# ls -lh /var/lib/mysql/thegeekstuff/EMPLOYEE.*
-rw-rw----. 1 mysql mysql   22G Apr 23 12:03 EMPLOYEE.MYD
-rw-rw----. 1 mysql mysql  500M Apr 23 14:10 EMPLOYEE.TMD

After the optimize table command finishes, you’ll not see the temporary table. Instead, you’ll see the original EMPLOYEE.MYD file that is optimized and with reduced file size.

# ls -lh /var/lib/mysql/thegeekstuff/EMPLOYEE.*
-rw-rw----. 1 mysql mysql    2G Apr 23 14:20 EMPLOYEE.MYD

3. Defrag using mysqlcheck command

The second method to optimize a table is using mysqlcheck command as shown below.

The following example will optimize the DEPARTMENT table. You’ll execute this command from the Linux prompt (and not on MySQL prompt).

# mysqlcheck -o thegeekstuff DEPARTMENT -u root -pMySQLSecretPwd99
thegeekstuff.DEPARTMENT  OK

Note: Internally mysqlcheck command uses “OPTIMIZE TABLE” command.

In the above example:

  • mysqlcheck is the command that is executed from the Linux prompt.
  • -o option is to indicate that mysqlcheck should perform “optimize table” operation.
  • thegeekstuff is the database
  • DEPARTMENT is the table inside thegeekstuff database that should be optimized
  • -u root indicates that mysqlcheck command should use “root” as the mysql user to connect
  • -p indicates the password for the root account of mysql. Please note that there is no space between -p option and the password.

Apart from optimization, you can also use mysqlcheck command to check, analyze and repair tables in your mysql database.

4. Defrag All Tables or All Databases

If you want to optimize all the tables in a particular MySQL database, use the following command.

The following command will optimize all the tables located in thegeekstuff database.

mysqlcheck -o thegeekstuff -u root -pMySQLSecretPwd99

If you have multiple database running on your system, you can optimize all the tables located under all the database on your system using the following command.

The following will optimize ALL database on your system.

mysqlcheck -o --all-databases -u root -pMySQLSecretPwd99

5. After Optimization

After the optimization, using the following query, check the total-size and unused-space-size for the three tables that we optimized in this example.

mysql> use thegeekstuff;

mysql> select table_name,
round(data_length/1024/1024) as data_length_mb, 
round(data_free/1024/1024) as data_free_mb 
 from information_schema.tables 
 where table_name in 
| table_name | data_length_mb | data_free_mb |
| BENEFITS   |           2968 |            0 |
| DEPARTMENT |            980 |            0 |
| EMPLOYEE   |           1799 |            0 |

As we see from the above output, the data_length_mb is drastically reduced for these tables after the optimization. Also, the data_free_mb is now at 0, as there are no fragmentation anymore.

The file size for these tables are now lot less when compared to the original size. We have now reclaimed lot of unused space at the filsystem level for these tables.

# ls -lh /var/lib/mysql/thegeekstuff/
-rw-rw----. 1 mysql mysql    3G Apr 23 14:23 BENEFITS.MYD
-rw-rw----. 1 mysql mysql  980M Apr 23 14:30 DEPARTMENT.MYD
-rw-rw----. 1 mysql mysql    2G Apr 23 14:45 EMPLOYEE.MYD

In this example, we’ve reclaimed around 37GB of unused space after optimizing these three tables.

Source: The Geek Stuff

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