MySQL creates temporary tables on disk. How do I stop it?
Looking at the my.ini, I have two suggestions
SUGGESTION #1
I would bump up the following settings in your my.ini
sort_buffer_size=4M
join_buffer_size=4M
This will make some joins and sort stay in memory. Of course, once a JOIN or an ORDER BY needs more than 4M, it will page to disk as a MyISAM table.
If you cannot login as root@localhost, then restart mysql with
C:\> net stop mysql
C:\> net start mysql
If you can login as root@localhost, you do not have to restart mysql to use these settings.
Just run this in the MySQL client:
SET @FourMegs = 1024 * 1024 * 4;
SET GLOBAL sort_buffer_size = @FourMegs;
SET GLOBAL join_buffer_size = @FourMegs;
SUGGESTION #2
Since your Data is on Drive D:, you may have Disk I/O on Drive C:.
Please run this query:
mysql> show variables like 'tmpdir';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tmpdir | C:\Windows\TEMP |
+---------------+-----------------+
1 row in set (0.00 sec)
Since I run mysql on my Desktop with defaults, my temp tables are being written to Drive C:. If Drive D is a better disk than Drive C:, perhaps you can map temp tables to Drive D: by setting tmpdir in my.ini as follows:
tmpdir="D:/DBs/"
You will have to restart mysql since tmpdir is not a dynamic variable.
Give it a Try !!!
UPDATE 2013-11-29 10:09 EST
SUGGESTION #3
Given the fact that MySQL is running in Windows and you cannot touch the queries in the core package, I have two ideas tat must be done together.
IDEA #1 : Move the Database to a Linux Machine
You should be able to
- Setup a Linux machine
- Install MySQL on the Linux machine
- Enable Binary Logging for MySQL in Windows
- mysqldump the database to a text SQL file
- Load SQL file to MySQL running in Linux
- Setup replication from MySQL/Windows to MySQL/Linux
IDEA #2 : Reconfigure Moodle to point to the Linux Machine
Moodle was designed for LAMP in the first place. Just change the config files to point to the Linux machine instead of localhost.
Here is a link to an old Moodle 2.3 doc on setting up MySQL : http://docs.moodle.org/23/en/Installing_Moodle#Create_an_empty_database
I am sure the latest docs are available as well.
What is the Point of Moving the Database to Linux ???
How does this help the temp table situation ???
I would then suggestion setting up a RAM disk as the target folder for your temp tables
Jan 04, 2013: Is there a MySQL engine or trick to avoid writing so many temp tables to disk?Dec 17, 2012: Why does MySQL produce so many temporary MYD files? (Actual instructions)Nov 30, 2012: Is it bad to create many mysql temporary tables simultaneously?
Temp table creation will still happen, but it will be written to RAM rather than disk. reducing Disk I/O.
UPDATE 2013-11-29 11:24 EST
SUGGESTION #4
I would suggest revisiting SUGGESTION #2 with a fast RAID-0 disk (32+ GB), configuring it as Drive T: (T for Temp). After installing such a disk, add this to my.ini:
[mysqld]
tmpdir="T:\"
MySQL restart would be required, using
net stop mysql
net start mysql
BTW I said RAID-0 on purpose so that you can get good write performance over a RAID-1, RAID-10. A tmp table disk is not something I would make redundant.
Without optimizing the queries as @RaymondNijland has been commenting on, you cannot reduce the temp table creation count in any way. SUGGESTION #3 and SUGGESTION #4 offer speeding up temp table creation and temp table I/O as the only alternative.
I answer my own question here for completeness
I will select @RolandoMySQLDBA as the preferred answer because it gave me the most hints even though it didn't actually solve my problem.
Below are the results of my investigation
Conclusion
MySQL on Windows just creates lots of temporary tables and tuning MySQL by modifying the content of the configuration files did not help.
Details
The table details the parameters I have modified in my.ini respectively before executing any queries. MySQL was re-started between each test.
I used the my.ini found in the original question as a template and I then changed the value of the parameters one by one according to the table below.
I used JMeter to generate 100 concurrent web requests (as that represented our usage) repeated 10 ten times. Each Test consisted thus of 1000 requests in total. This resulted in subsequent database calls. This showed that MySQL would create lots of temporary tables regardless of what configuration parameters we changed.
+----+------------+-------+---------------+------------+
|Test|Parameter |Value |NumOfTempTables|Db Max Conn |
+----+------------+-------+---------------+------------+
| 1 |key_buffer_ | 25M | 30682 | 29 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 2 |key_buffer_ | 55M | 30793 | 29 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 3 |key_buffer_ | 100M | 30666 | 28 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 4 |key_buffer_ | 125M | 30593 | 24 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 5 |query_cache_| 100M | 30627 | 32 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 6 |query_cache_| 250M | 30761 | 26 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 7 |query_cache_| 500M | 30864 | 83* |
| |size | | | |
+----+------------+-------+---------------+------------+
| 8 |query_cache_| 1G | 30706 | 75* |
| |size | | | |
+----+------------+-------+---------------+------------+
| 9 |tmp_table_ | 125M | 30724 | 31 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 10 |tmp_table_ | 250M | 30689 | 90* |
| |size | | | |
+----+------------+-------+---------------+------------+
| 11 |tmp_table_ | 500M | 30792 | 28 |
| |size | | | |
+----+------------+-------+---------------+------------+
| 12 |Sort_buffer&| 256K | 30754 | 28 |
| |Join_buffer | | | |
+----+------------+-------+---------------+------------+
| 13 |Sort_buffer&| 512K | 30788 | 30 |
| |Join_buffer | | | |
+----+------------+-------+---------------+------------+
| 14 |Sort_buffer&| 1M | 30788 | 28 |
| |Join_buffer | | | |
+----+------------+-------+---------------+------------+
| 15 |Sort_buffer&| 4M | 30642 | 35 |
| |Join_buffer | | | |
+----+------------+-------+---------------+------------+
| 16 |innodb- | 1G | 30695 | 33 |
| |buffer- | | | |
| |pool-size | | | |
+----+------------+-------+---------------+------------+
| 17 |innodb- | 2G | 30791 | 28 |
| |buffer- | | | |
| |pool-size | | | |
+----+------------+-------+---------------+------------+
| 18 |innodb- | 3G | 30719 | 34 |
| |buffer- | | | |
| |pool-size | | | |
+----+------------+-------+---------------+------------+
*Average of three runs
The images below depict the amount of memory and CPU the database server required for the different configurations. The black lines indicate the minimum and maximum values and the blue bars indicate start and end values. The maximum memory was 4096M as indicated in the question.
