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
InnoDB:
In sequential read-ahead, if InnoDB
notices 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 InnoDB notices
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
called doublewrite. It adds safety to
recovery following an operating system crash or a power outage,
and improves performance on most varieties of Unix by reducing
the need for fsync() operations.
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 the operating system crashes in
the middle of a page write, InnoDB can later
find a good copy of the page from the doublewrite buffer during
recovery.
