Oracle 5.0, Reference Manual

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The
MyISAM
Key Cache
700
key_buffer_size
[458] is the cache component. See Section 5.1.5.1, “Structured System
Variables” , for a description of the syntax used for referring to structured key cache system variables.
By default, table indexes are assigned to the main (default) key cache created at the server startup.
When a key cache is destroyed, all indexes assigned to it are reassigned to the default key cache.
For a busy server, you can use a strategy that involves three key caches:
• A “hot” key cache that takes up 20% of the space allocated for all key caches. Use this for tables that
are heavily used for searches but that are not updated.
• A “cold” key cache that takes up 20% of the space allocated for all key caches. Use this cache for
medium-sized, intensively modified tables, such as temporary tables.
• A “warm” key cache that takes up 60% of the key cache space. Employ this as the default key cache,
to be used by default for all other tables.
One reason the use of three key caches is beneficial is that access to one key cache structure does not
block access to the others. Statements that access tables assigned to one cache do not compete with
statements that access tables assigned to another cache. Performance gains occur for other reasons
as well:
• The hot cache is used only for retrieval queries, so its contents are never modified. Consequently,
whenever an index block needs to be pulled in from disk, the contents of the cache block chosen for
replacement need not be flushed first.
• For an index assigned to the hot cache, if there are no queries requiring an index scan, there is a
high probability that the index blocks corresponding to nonleaf nodes of the index B-tree remain in
the cache.
• An update operation most frequently executed for temporary tables is performed much faster when
the updated node is in the cache and need not be read in from disk first. If the size of the indexes of
the temporary tables are comparable with the size of cold key cache, the probability is very high that
the updated node is in the cache.
The
CACHE INDEX
statement sets up an association between a table and a key cache, but the
association is lost each time the server restarts. If you want the association to take effect each time the
server starts, one way to accomplish this is to use an option file: Include variable settings that configure
your key caches, and an
init-file
option that names a file containing
CACHE INDEX
statements to
be executed. For example:
key_buffer_size = 4G
hot_cache.key_buffer_size = 2G
cold_cache.key_buffer_size = 2G
init_file=/ path / to / data-directory /mysqld_init.sql
The statements in
mysqld_init.sql
are executed each time the server starts. The file should
contain one SQL statement per line. The following example assigns several tables each to
hot_cache
and
cold_cache
:
CACHE INDEX db1.t1, db1.t2, db2.t3 IN hot_cache
CACHE INDEX db1.t4, db2.t5, db2.t6 IN cold_cache
8.6.1.3. Midpoint Insertion Strategy
By default, the key cache management system uses a simple LRU strategy for choosing key cache
blocks to be evicted, but it also supports a more sophisticated method called the midpoint insertion
strategy.
When using the midpoint insertion strategy, the LRU chain is divided into two parts: a
hot sublist and a warm sublist. The division point between two parts is not fixed, but the
key cache management system takes care that the warm part is not “too short,” always