(mysql.info.gz) Midpoint insertion

(mysql.info.gz) Multiple key caches
 
 7.4.6.3 Midpoint Insertion Strategy
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 By default, the key cache management system of MySQL 4.1 uses the 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 sub-chain and a warm sub-chain. 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
 containing at least `key_cache_division_limit' percent of the key cache
 blocks.  `key_cache_division_limit' is a component of structured key
 cache variables, so its value is a parameter that can be set per cache.
 
 When an index block is read from a table into the key cache, it is
 placed at the end of the warm sub-chain. After a certain number of hits
 (accesses of the block), it is promoted to the hot sub-chain.  At
 present, the number of hits required to promote a block (3) is the same
 for all index blocks.  In the future, we will allow the hit count to
 depend on the B-tree level of the node corresponding to an index block:
 Fewer hits will be required for promotion of an index block if it
 contains a non-leaf node from the upper levels of the index B-tree than
 if it contains a leaf node.
 
 A block promoted into the hot sub-chain is placed at the end of the
 chain.  The block then circulates within this sub-chain. If the block
 stays at the beginning of the sub-chain for a long enough time, it is
 demoted to the warm chain. This time is determined by the value of the
 `key_cache_age_threshold' component of the key cache.
 
 The threshold value prescribes that, for a key cache containing N
 blocks, the block at the beginning of the hot sub-chain not accessed
 within the last `N*key_cache_age_threshold/100' hits is to be moved to
 the beginning of the warm sub-chain. It then becomes the first
 candidate for eviction, because blocks for replacement always are taken
 from the beginning of the warm sub-chain.
 
 The midpoint insertion strategy allows you to keep more-valued blocks
 always in the cache. If you prefer to use the plain LRU strategy, leave
 the `key_cache_division_limit' value set to its default of 100.
 
 The midpoint insertion strategy helps to improve performance when
 execution of a query that requires an index scan effectively pushes out
 of the cache all the index blocks corresponding to valuable high-level
 B-tree nodes. To avoid this, you must use a midpoint insertion strategy
 with the `key_cache_division_limit' set to much less than 100.  Then
 valuable frequently hit nodes will be preserved in the hot sub-chain
 during an index scan operation as well.
Info Catalog
(mysql.info.gz) Multiple key caches
(mysql.info.gz) MyISAM key cache
(mysql.info.gz) Index preloading
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