The doublewrite buffer is a storage area located in the system
InnoDB writes pages that are
flushed from the
InnoDB buffer pool, before the
pages are written to their proper positions in the data file. Only
after flushing and writing pages to the doublewrite buffer, does
InnoDB write pages to their proper positions.
If there is an operating system, storage subsystem, or
mysqld process crash in the middle of a page
InnoDB can later find a good copy of the
page from the doublewrite buffer during crash recovery.
Although data is always written twice, the doublewrite buffer does
not require twice as much I/O overhead or twice as many I/O
operations. Data is written to the doublewrite buffer itself as a
large sequential chunk, with a single
call to the operating system.
The doublewrite buffer is enabled by default. To disable the
doublewrite buffer, set
innodb_doublewrite to 0.