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.