We are running a site (Moodle) that the users currently find slow. I think I have tracked down the problem to MySQL creating temporary tables on disk. I watch the variable created_tmp_disk_tables
in Mysql Workbench server administration and the number increases with roughly 50 tables/s. After a days usage, created_tmp_disk_tables
is >100k. Also, the memory does not seem to be released. The usage keeps increasing until the system becomes pretty much unusable and we have to re-start MySQL. I need to re-start it almost every day and it begins with using about 30-35% of available memory and finishing the day with 80%.
I have no blobs in the database and no control over the queries either so I can't attempt to optimise them. I have also used the Percona Confirguration Wizard to generate a configuration file but that my.ini didn't solve my problem either.
Questions
-
What should I change to stop MySQL from creating temporary tables on disk? Are there settings I need to change? Should I throw more memory at it?
-
How can I stop MySQL from eating up my memory?
Edit
I enabled slow_queries
log and discovered that the query SELECT GET_LOCK()
was logged as slow. A quick search revealed that I had allowed persistent connections in the PHP configuration (mysqli.allow_persistent = ON
). I turned this off. This reduced the rate at which MySQL consumes memory.It is still creating temporary tables though.
I also checked that the key_buffer size
is large enough.
I looked at the variable key_writes
. This should be zero. If not, increase the key_buffer_size
.I have zero key_reads
and zero key_writes
so I assume that the key_buffer_size
is large enough.
I increased the tmp_table_size
and max-heap-table-size
to 1024M as an increase in created_tmp_disk_tables may indicate that the tables can't fit in memory. This didn't solve it.
Edit 2
If you see many sort_merge_passes
per second in SHOW GLOBAL STATUS output, you can consider increasing the sort_buffer_size
value. I had 2 sort_merge_passes
in an hour so I consider the sort_buffer_size
to be large enough.
Ref: Mysql Manual on sort_buffer_size
Edit 3
I have modified the sort and join buffers as suggested by @RolandoMySQLDBA. The result is displayed in the table below but I think the created_tmp_tables_on_disk
is still high. I restarted the mysql server after I changed the value and checked the created_tmp_tables_on_disk
after a day (8h) and calculated the average. Any other suggestions? It seems to me that there is something that doesn't fit inside some kind of container but I can't work out what it is.
+---------------------+-------------+-------------+--------------------+
| Tmp_table_size, | Sort_buffer | Join_buffer | No of created |
| max_heap_table_size | | | tmp_tables on disk |
+---------------------+-------------+-------------+--------------------+
| 125M | 256K | 256K | 100k/h |
+---------------------+-------------+-------------+--------------------+
| 125M | 512K | 512K | 100k/h |
+---------------------+-------------+-------------+--------------------+
| 125M | 1M | 1M | 100k/h |
+---------------------+-------------+-------------+--------------------+
| 125M | 4M | 4M | 100k/h |
+---------------------+-------------+-------------+--------------------+
This is my configuration:
+-----------------------+-----------------------+
|DATABASE SERVER |WEB SERVER |
+-----------------------+-----------------------+
|Windows Server 2008 R2 |Windows Server 2008 R2 |
+-----------------------+-----------------------+
|MySQL 5.1.48 |IIS 7.5 |
+-----------------------+-----------------------+
|4 Core CPU |4 Core CPU |
+-----------------------+-----------------------+
|4GB RAM |8GB RAM |
+-----------------------+-----------------------+
Additional information
+--------------------+---------+
|PARAM |VALUE |
+--------------------+---------+
|Num of tables in Db |361 |
+--------------------+---------+
|Size of database |2.5G |
+--------------------+---------+
|Database engine |InnoDB |
+--------------------+---------+
|Read/write ratio |3.5 |
|(Innodb_data_read/ | |
|innodb_data_written)| |
+--------------------+---------+
|Avg table size |15k rows |
+--------------------+---------+
|Max table size |744k rows|
+--------------------+---------+
This setup was given to me so I have limited control over it. The web server is using very little CPU and RAM so I have excluded that machine as a bottleneck. A majority of the MySQL settings originates from a config auto-generation tool.
I have monitored the system using PerfMon over a few representative days. From that, I conclude that it is not the OS that is swapping to disk.
My.ini
[client]
port=3306
[mysql]
default-character-set=utf8
[mysqld]
port=3306
basedir="C:/Program Files/MySQL/MySQL Server 5.1/"
datadir="D:/DBs/Data/"
default-character-set=utf8
default-storage-engine=INNODB
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
max_connections=125
query_cache_size=350M
table_cache=1520
tmp_table_size=125M
table-definition-cache= 1024
max-heap-table-size= 32M
thread_cache_size=38
MyISAM Specific options
myisam_max_sort_file_size=100G
myisam_sort_buffer_size=125M
key_buffer_size=55M
read_buffer_size=1024K
read_rnd_buffer_size=256K
sort_buffer_size=1024K
join_buffer_size=1024K
INNODB Specific options
innodb_data_home_dir="D:/DBs/"
innodb_additional_mem_pool_size=32M
innodb_flush_log_at_trx_commit=1
innodb_log_buffer_size=16M
innodb_buffer_pool_size=2G
innodb_log_file_size=407M
innodb_thread_concurrency=8
Best Answer
Looking at the
my.ini
, I have two suggestionsSUGGESTION #1
I would bump up the following settings in your
my.ini
This will make some joins and sort stay in memory. Of course, once a
JOIN
or anORDER BY
needs more than4M
, it will page to disk as a MyISAM table.If you cannot login as
root@localhost
, then restart mysql withIf you can login as root@localhost, you do not have to restart mysql to use these settings.
Just run this in the MySQL client:
SUGGESTION #2
Since your Data is on Drive
D:
, you may have Disk I/O on DriveC:
.Please run this query:
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 DriveC:
, perhaps you can map temp tables to DriveD:
by setting tmpdir inmy.ini
as follows: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
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
:MySQL restart would be required, using
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
andSUGGESTION #4
offer speeding up temp table creation and temp table I/O as the only alternative.