FogBugz speed issue – an example of MySQL slow queries troubleshooting

We are using FogBugz application for the bugs and support requests tracking. And it’s configured to receive [email protected] emails (means tons of spam). Once the database size hit approximately 20,000 requests, we started really wonder: why so slow? It looks like there was another table BugEvent which size is around 200,000 records at that time.

It could be some other application, could be your own code. Doesn’t matter – just it happens, one database size grows, performance of the application is degrading.

First step to troubleshoot – enable slow queries log in the MySQL config file (/etc/my.cnf is popular location). Open file in editor and add lines

log_slow_queries        = /var/log/mysql-slow.log
long_query_time = 10

(path for the log file is totally up to you, I use MySQL 5.0*, so in other versions syntax can be different)

Those lines tell mysql to log all queries lasted 10 seconds or more. Restart MySQL now and wait.

After a while run  mysqldumpslow as root. If there are any slow queries logged, you will see summary, here is mine:

Reading mysql slow query log from /var/log/mysql/mysql-slow.log

Count: 36  Time=549.81s (19793s)  Lock=0.00s (0s)  Rows=0.0 (0)
SELECT Bug.ixBug AS ix, Bug.ixBugEventLatest AS ixChild,
       Area.nTypeAs nAreaType
FROM (Bug INNER JOIN Area ON Bug.ixArea = Area.ixArea)
WHERE ixBugEventLatest  <= N
       AND ixBugEventLatest >= N AND Bug.ixBug IN
          (SELECT ix FROM IndexDelta WHERE sType = 'S'
           AND fDeleted = N)
       AND -N =  -N
ORDER BY ixBugEventLatest  DESC  

Query listed above appears to take around 10 minutes to execute. Now check the tables type in the MySQL

mysql> show table status;

In our scenario I found that all tables are MyISAM. That means, that while query is executed (10 minutes) tables used in it will be locked for writing. So, if someone tries to update bug, they have to wait up to 10 minutes.

Check what MySQL engines you have:

mysql> show engines;

| Engine  | Support  | Comment                  
| MyISAM  | DEFAULT  | Default engine as of MySQL
| MEMORY  | YES      | Hash based, stored in memory
| InnoDB  | YES      | Supports transactions, row-

If InnoDB is supported, then all what has to be done to speed up application is switching to InnoDB in this situation. This will not make query above run any faster, but, that will not lock the whole table for the time query is running. There are some benefits of MyISAM table type (well, it’s faster!), so you may don’t want to convert all tables to InnoDB at this point. But large tables with concurrent updates and reads will benefit.

If InnoDB is not supported, it can be turned on in MySQL configuration file, just comment out skip-innodb line.

In case of FogBugz I found the following tables to be worst offenders:
Bug, BugEvent, BugRelation, SorterToken, TokenAssociation.

To convert those, use:

mysql> alter table Bug engine=InnoDB;

After conversion performance of the application increased immediately, as there are no more locks on popular tables.

There are additional benefits of InnoDB engine, such as foreign keys. Read more on InnoDB: