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 file flush technique called doublewrite.
- 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.