MyISAM Key buffer命中率：
100 - ( (Key_reads * 100) / Key_read_requests )
MyISAM Key buffer的使用率：
100 - ( (Key_blocks_unused * key_cache_block_size) * 100 / key_buffer_size )
InnoDB Buffer Pool 的命中率
(Innodb_buffer_pool_read_requests - Innodb_buffer_pool_reads) / Innodb_buffer_pool_read_requests * 100%[@more@]
当前题目：计算mysqlbuffer的命中率及使用率
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