13.7.8.8. More Read-Ahead Statistics

As described in Section 13.7.7.7, “Changes in the Read-Ahead Algorithm” a read-ahead request is an asynchronous I/O request issued in anticipation that a page will be used in the near future. Knowing how many pages are read through this read-ahead mechanism, and how many of them are evicted from the buffer pool without ever being accessed, can be useful to help fine-tune the parameter innodb_read_ahead_threshold.

Starting from InnoDB 1.0.5, SHOW STATUS output displays the global status variables Innodb_buffer_pool_read_ahead and Innodb_buffer_pool_read_ahead_evicted. These variables indicate the number of pages read in as part of read ahead, and the number of such pages evicted without ever being accessed respectively. These counters provide global values since the start of the server. Note that the status variables Innodb_buffer_pool_read_ahead_rnd and Innodb_buffer_pool_read_ahead_seq have been removed from the SHOW STATUS output.

In addition to the two counters mentioned above, SHOW INNODB STATUS also shows the rate at which the read-ahead pages are read in and the rate at which such pages are evicted without being accessed. The per-second averages are based on the statistics collected since the last invocation of SHOW INNODB STATUS and are displayed in the BUFFER POOL AND MEMORY section of the output.

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