MySQL Reference Manual for version 4.0.18.

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7.4.6.5 Key Cache Block Size

MySQL 4.1 introduces a new key_cache_block_size variable on a per-key cache basis. This variable specifies the size of the block buffers for a key cache.

This variable is introduced to allow tuning the performance of I/O operations for index files.

The best performance for I/O operations is achieved when the size of read buffers are equal to the size of the native operating system I/O buffers. But setting the size of key nodes equal to the size of I/O buffer does not always ensure the best overall performance. When reading the big leaf nodes the server pulls in a lot of unnecessary data, effectively preventing reading other leaf nodes.

Currently, you cannot control the size of the index blocks in a table. This size is set by the server when the `.MYI' index file is created, depending on the size of the keys in the indexes present in the table definition. In most cases, it is set equal to the I/O buffer size. In the future this will be changed and then key_cache_block_size variable will be fully employed.



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