This section describes issues that you may encounter when using
the InnoDB
memcached plugin.
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 thedaemon_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 thedaemon_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 thedaemon_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 theinnodb_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 theinnodb_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 thedaemon_memcached_option
configuration parameter to specify a unique memcached port for eachdaemon_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 theInnoDB
table in theinnodb_memcache.containers
table, or you may have not switched to the correctInnoDB
table by issuing aget
orset
request using@@
notation. This problem could also occur if you change an existing entry in thetable_id
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 ascol1|col2|col3
may still work, even if the daemon is using thetest.demo_test
table which stores values in a single column.When defining your own
InnoDB
table for use with thedaemon_memcached
plugin, and columns in the table are defined asNOT NULL
, ensure that values are supplied for theNOT NULL
columns when inserting a record for the table into theinnodb_memcache.containers
table. If theINSERT
statement for theinnodb_memcache.containers
record contains fewer delimited values than there are mapped columns, unfilled columns are set toNULL
. Attempting to insert aNULL
value into aNOT NULL
column causes theINSERT
to fail, which may only become evident after you reinitialize thedaemon_memcached
plugin to apply changes to theinnodb_memcache.containers
table.If
cas_column
andexpire_time_column
fields of theinnodb_memcached.containers
table are set toNULL
, 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 thecas_column
andexpire_time_column
columns. Set the value of these columns to0
when the columns are unused.As the length of the memcached key and values increase, you might encounter size and length limits.
When the key exceeds 250 bytes, memcached operations return an error. This is currently a fixed limit within memcached.
InnoDB
table limits may be encountered if values exceed 768 bytes in size, 3072 bytes in size, or half of theinnodb_page_size
value. These limits primarily apply if you intend to create an index on a value column to run report-generating queries on that column using SQL. See Section 17.22, “InnoDB Limits” for details.The maximum size for the key-value combination is 1 MB.
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 theloose
prefix with option names. For example, useloose-daemon_memcached_option='-c 64'
instead ofdaemon_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.