MySQL Manual
7 MySQL Table Types
7.5.12.1 Disk I/O
In disk I/O InnoDB uses asynchronous I/O. On Windows NT it uses the native asynchronous I/O provided by the operating system. On Unix, InnoDB uses simulated asynchronous I/O built into InnoDB: InnoDB creates a number of I/O threads to take care of I/O operations, such as read-ahead. In a future version we will add support for simulated aio on Windows NT and native aio on those versions of Unix which have one.
On Windows NT InnoDB uses non-buffered I/O. That means that the disk pages InnoDB reads or writes are not buffered in the operating system file cache. This saves some memory bandwidth.
Starting from 3.23.41 InnoDB uses a novel file flush technique called doublewrite. It adds safety to crash recovery after an operating system crash or a power outage, and improves performance on most Unix flavors by reducing the need for fsync operations.
Doublewrite means that InnoDB before writing pages to a datafile 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, InnoDB writes the pages to their proper positions in the datafile. If the operating system crashes in the middle of a page write, InnoDB will in recovery find a good copy of the page from the doublewrite buffer.
Starting from 3.23.41
you can also use a raw disk partition as a datafile, though this has
not been tested yet. When you create a new datafile you have
to put the keyword newraw immediately after the datafile size
in innodb_data_file_path. The partition must be at least as large
as the size that you specify. Note that 1M in InnoDB is
1024 x 1024 bytes, while in disk specifications 1 MB usually means
1000 000 bytes.
innodb_data_file_path=/dev/hdd1:5Gnewraw;/dev/hdd2:2Gnewraw
When you start the database again you must change the keyword
to raw. Otherwise, InnoDB will write over your
partition!
innodb_data_file_path=/dev/hdd1:5Graw;/dev/hdd2:2Graw
By using a raw disk you can on some versions of Unix perform unbuffered I/O.
When you use raw disk partitions, make sure they have permissions that allow read and write access to the account used for running the MySQL server.
There are two read-ahead heuristics in InnoDB: sequential read-ahead and random read-ahead. In sequential read-ahead InnoDB notices that the access pattern to a segment in the tablespace is sequential. Then InnoDB will post in advance a batch of reads of database pages to the I/O system. In random read-ahead InnoDB notices that some area in a tablespace seems to be in the process of being fully read into the buffer pool. Then InnoDB posts the remaining reads to the I/O system.
