Innodb_buffer_pool_pages_free 如果＝０，pool严重不足.
命中率计算：
(Innodb_buffer_pool_read_requests-Innodb_buffer_pool_reads)/Innodb_buffer_pool_read_requests
Innodb_buffer_pool_wait_free 为等待释放内存的次数，如果>0 证明pool不足.
SHOW GLOBAL STATUS LIKE 'Innodb_buffer_pool_%'; 查看相关参数..