InnoDB uses simulated asynchronous disk I/O:
InnoDB creates a number of threads to take care
of I/O operations, such as read-ahead.
There are two read-ahead heuristics in
In sequential read-ahead, if
InnoDBnotices that the access pattern to a segment in the tablespace is sequential, it posts in advance a batch of reads of database pages to the I/O system.
In random read-ahead, if
InnoDBnotices that some area in a tablespace seems to be in the process of being fully read into the buffer pool, it posts the remaining reads to the I/O system.
InnoDB uses a novel file flush technique
involving a structure called the
which is enabled by default
innodb_doublewrite=ON). It adds
safety to recovery following a crash or power outage, and improves
performance on most varieties of Unix by reducing the need for
Doublewrite means that before writing pages to a data file,
InnoDB first writes them to a contiguous
tablespace area called the doublewrite buffer. Only after the
write and the flush to the doublewrite buffer has completed does
InnoDB write the pages to their proper
positions in the data file. If there is an operating system,
storage subsystem, or mysqld process crash in
the middle of a page write (causing a
torn page condition),
InnoDB can later find a good copy of the page
from the doublewrite buffer during recovery.