If you encounter the following error in the MySQL error log, the server might fail to start:

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

The error message is from the memcached daemon. One solution is to raise the OS limit for the number of open files. The commands for checking and increasing the open file limit varies by operating system. This example shows commands for Linux and macOS:

# Linux
$> ulimit -n
1024
$> ulimit -n 4096
$> ulimit -n
4096

# macOS
$> ulimit -n
256
$> ulimit -n 4096
$> ulimit -n
4096

The other solution is to reduce the number of concurrent connections permitted for the memcached daemon. To do so, encode the -c memcached option in the daemon_memcached_option configuration parameter in the MySQL configuration file. The -c option has a default value of 1024.

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

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

[mysqld]
...
loose-daemon_memcached_option='-vvv'

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

If the daemon_memcached plugin causes MySQL server startup issues, you can temporarily disable the daemon_memcached plugin while troubleshooting by adding this line under the [mysqld] group in the MySQL configuration file:

daemon_memcached=OFF

For example, if you run the INSTALL PLUGIN statement before running the innodb_memcached_config.sql configuration script to set up the necessary database and tables, the server might unexpectedly exit and fail to start. The server could also fail to start if you incorrectly configure an entry in the innodb_memcache.containers table.

To uninstall the memcached plugin for a MySQL instance, issue the following statement:

mysql> UNINSTALL PLUGIN daemon_memcached;

If you run more than one instance of MySQL on the same machine with the daemon_memcached plugin enabled in each instance, use the daemon_memcached_option configuration parameter to specify a unique memcached port for each daemon_memcached plugin.

If an SQL statement cannot find the InnoDB table or finds no data in the table, but memcached API calls retrieve the expected data, you may be missing an entry for the InnoDB table in the innodb_memcache.containers table, or you may have not switched to the correct InnoDB table by issuing a get or set request using @@table_id notation. This problem could also occur if you change an existing entry in the innodb_memcache.containers table 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 such as col1|col2|col3 may still work, even if the daemon is using the test.demo_test table which stores values in a single column.

When defining your own InnoDB table for use with the daemon_memcached plugin, and columns in the table are defined as NOT NULL, ensure that values are supplied for the NOT NULL columns when inserting a record for the table into the innodb_memcache.containers table. If the INSERT statement for the innodb_memcache.containers record contains fewer delimited values than there are mapped columns, unfilled columns are set to NULL. Attempting to insert a NULL value into a NOT NULL column causes the INSERT to fail, which may only become evident after you reinitialize the daemon_memcached plugin to apply changes to the innodb_memcache.containers table.

If cas_column and expire_time_column fields of the innodb_memcached.containers table are set to NULL, the following error is returned when attempting to load the memcached plugin:

InnoDB_Memcached: column 6 in the entry for config table 'containers' in
database 'innodb_memcache' has an invalid NULL value.

The memcached plugin rejects usage of NULL in the cas_column and expire_time_column columns. Set the value of these columns to 0 when the columns are unused.

As the length of the memcached key and values increase, you might encounter size and length limits.

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

There is no restriction or check in place to validate character set settings. memcached stores and retrieves keys and values in bytes and is therefore not character set sensitive. However, you must ensure that the memcached client and the MySQL table use the same character set.

memcached connections are blocked from accessing tables that contain an indexed virtual column. Accessing an indexed virtual column requires a callback to the server, but a memcached connection does not have access to the server code.