计算mysql buffer的命中率及使用率

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%