innodb 内存评估原则
通常InnoDB存储引擎的缓冲池的命中率不应该小于99%。
缓冲池命中率 = (Innodb_buffer_pool_read_requests)/(Innodb_buffer_pool_read_requests + Innodb_buffer_pool_read_ahead + Innodb_buffer_pool_reads)
innodb_buffer_pool_read_requests
命中率 =-------------------------------------------------------------------------------------------------
(innodb_bufer_pool_read_requests + innodb_buffer_pool_read_ahead + innodb_buffer_pool_reads )
平均每次读取的字节数 = Innodb_data_read/Innodb_data_reads
Innodb_buffer_pool_reads：表示从物理磁盘读取的页数
Innodb_buffer_pool_read_ahead：预读的页数
Innodb_buffer_pool_read_ahead_evicted：预读的页数，但是没有被读取就从缓冲池中被替换的页的数量，一般用来判断预读的效率。
Innodb_buffer_pool_read_requests：从缓冲池中读取的次数。
Innodb_data_read：总共读入的字节数。
Innodb_data_reads：发起读请求的次数，每次读取可能需要读取多个页。
mysql> show global status like 'innodb%read%';
Innodb_buffer_pool_read_ahead_rnd 0
Innodb_buffer_pool_read_ahead 651270
Innodb_buffer_pool_read_ahead_evicted 792
Innodb_buffer_pool_read_requests 6652087462
Innodb_buffer_pool_reads 1643398
Innodb_data_pending_reads 0
Innodb_data_read 42994913280
Innodb_data_reads 2635909
Innodb_pages_read 2624075
Innodb_rows_read 6299272374
innodb_buffer_pool_read_requests
命中率 =-------------------------------------------------------------------------------------------------
(innodb_bufer_pool_read_requests + innodb_buffer_pool_read_ahead + innodb_buffer_pool_reads )
6644810356
命中率 = ---------------------------- = 99.96% 约接近 1 越好 (证明内存足够)
6644810356 + 651270 + 1643325