InnoDB uses asynchronous disk I/O where possible, by creating a number of threads to
handle I/O operations, while permitting other database operations to proceed while the I/O is still in progress.
On Linux and Windows platforms, InnoDB uses the available OS and library functions to perform "native" asynchronous I/O. On other platforms, InnoDB
still uses I/O threads, but the threads may actually wait for I/O requests to complete; this technique is known
as "simulated" asynchronous I/O.
If InnoDB can determine there is a high probability that data might be needed soon, it performs read-ahead operations to bring that data into the buffer pool so that it is available in memory. Making a few large read requests for contiguous data can be more efficient than making several small, spread-out requests. 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 involving a structure called the doublewrite buffer.
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
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
InnoDB write the pages to their proper positions in the data file.
If the operating system crashes in the middle of a page write (causing a torn page
InnoDB can later find a good copy of the page from the doublewrite
buffer during recovery.