(mysql.info) innodb-disk-io

(mysql.info) file-space-management

 
 14.2.14.1 Disk I/O
 ..................
 
 `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.

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