MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
The doublewrite buffer is a storage area where
InnoDB
writes pages flushed from the buffer
pool before writing the pages to their proper positions in the
InnoDB
data files. If there is an operating
system, storage subsystem, or unexpected mysqld
process exit in the middle of a page write,
InnoDB
can find a good copy of the page from
the doublewrite buffer during crash recovery.
Although data is 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 in a large
sequential chunk, with a single fsync()
call to
the operating system (except in the case that
innodb_flush_method
is set to
O_DIRECT_NO_FSYNC
).
The doublewrite buffer is enabled by default. To disable the
doublewrite buffer, set
innodb_doublewrite
to 0.