We already blog a lot on temporary tables on disk, how this could be bad for your workload and should be avoid.  Each temporary table will open a file descriptor,  external kernel call and by nature a well know file system slow operation.

We can point this benchmark simulating a working mail server


Monitoring of such queries can be trace via the status of

created_tmp_disk_tables 
Evidence of what is happening watching many Aria or MyISAM tables created on disk :

/usr/sbin/lsof  | grep "mysql" | grep "#"
mysqld     1855     mysql 1658u      REG                8,1       8192    1505932 …

In my previous post, I’ve showed how to use Dtrace to find queries creating disk temporary tables (only available for OS with dtrace: solaris, freebsd, etc).

In MySQL 5.6 (which is not released yet, use “labs” version for now) we can use new performance_schema table events_statements_history or events_statements_history_long to find all performance metrics for all queries including created disk/memory tables, use of index, etc. WOW! This is what I have been waiting for a long time!

To illustrate, I have grabbed mysql-5.6.3-labs-performance-schema-linux2.6-x86_64.tar.gz from labs.mysql.com (this feature is only in labs version) and run sysbench readonly test (you need to disable prepared statements in sysbench, seems to be not working with …