• key_cache_division_limit 

• key_cache_age_threshold 

This section describes the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 8.10.2, “The MyISAM Key Cache” . 

To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name  format. Examples: 

hot_cache.key_buffer_size hot_cache.key_cache_block_size cold_cache.key_cache_block_size 

For each structured system variable, an instance with the name of default  is always predefined. If you refer to a component of a structured variable without any instance name, the default  instance is used. Thus, default.key_buffer_size  and key_buffer_size  both refer to the same system variable. 

Structured variable instances and components follow these naming rules: 

•For a given type of structured variable, each instance must have a name that is unique within  variables of that type. However, instance names need not be unique across  structured variable types. For example, each structured variable has an instance named default , so default  is not unique across variable types. 

•The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name. 

•If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache  is not legal, but ‘hot-cache‘  is. 

• global , session , and local  are not legal instance names. This avoids a conflict with notation such as @@GLOBAL. var_name  for referring to nonstructured system variables. 

Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules may assume greater significance if some other type of structured variable is created in the future. 

With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option: 

shell> mysqld --hot_cache.key_buffer_size=64K 

In an option file, use this syntax: 

[mysqld] hot_cache.key_buffer_size=64K 

If you start the server with this option, it creates a key cache named hot_cache  with a size of 64KB in addition to the default key cache that has a default size of 8MB. 

Suppose that you start the server as follows: 

shell> mysqld --key_buffer_size=256K \ --extra_cache.key_buffer_size=128K \ 

