14.2.9.8. Troubleshooting the InnoDB memcached Plugin

If you see this error in your MySQL error log, the server might fail to start:

failed to set rlimit for open files. Try running as root or requestingsmaller maxconns value.

The error message is actually from the memcached daemon. One solution is to raise the OS limit for the number of open files. The command varies depending on the operating system. For example, here are the commands to check and increase the limit on several operating systems:

# Linux$ ulimit -n1024ulimit -n 4096$ ulimit -n4096# OS X Lion (10.6)$ ulimit -n256ulimit -n 4096$ ulimit -n4096

The other solution is to reduce the number of concurrent connections available for the memcached daemon, using the -c option which defaults to 1024. Encode that memcached option using the MySQL option daemon_memcached_option inside the MySQL configuration file:

[mysqld]...loose-daemon_memcached_option='-c 64'

To troubleshoot problems where the memcached daemon is unable to store data in or retrieve data from the InnoDB table, specify the memcached option -vvv through the MySQL configuration option daemon_memcached_option. Examine the MySQL error log for debug output related to memcached operations.

If the column specified to hold the memcached item values is the wrong data type, such as a numeric type instead of a string type, attempts to store key/value pairs will fail with no specific error code or message.

If the daemon_memcached plugin causes any issues with starting the MySQL server, disable it during troubleshooting by adding this line under the [mysqld] group in your MySQL configuration file:

daemon_memcached=OFF

For example, if you run the install plugin command before running the scripts/innodb_memcached_config.sql script to set up the necessary database and tables, the server might crash and be unable to start. Or, if you set up an incorrect entry in the innodb_memcache.containers table, the server might be unable to start.

To permanently turn off the memcached plugin for a MySQL instance, issue the following command:

mysql> uninstall plugin daemon_memcached;

If you run more than one instance of MySQL on the same machine, with the memcached daemon plugin enabled in each, make sure to specify a unique memcached port for each one using the daemon_memcached_option configuration option.

You might find that a SQL statement cannot find an expected table, or there is no data in the table, but memcached API calls still work and retrieve the expected data. This can happen if you do not set up the entry in the innodb_memcache.containers table, or do not switch to that table by issuing a GET or SET request with the key @@table_id, or make a change to an existing entry in innodb_memcache.containers without restarting the MySQL server afterward. The free-form storage mechanism is flexible enough that your requests to store or retrieve a multi-column value like col1|col2|col3 will usually still work, even if the daemon is using the test.demo_test table which stores all the data within a single column.

As the length of the memcached key and values increase, you encounter size and length limits at different points:

If you share configuration files across MySQL servers with different versions, using the latest configuration options for the memcached plugin could cause startup errors for older MySQL versions. To avoid compatibility problems, use the loose forms of these option names, for example loose-daemon_memcached_option='-c 64' instead of daemon_memcached_option='-c 64'.