MySQL 5.7 Release Notes
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 in most cases. To
disable the doublewrite buffer, set
innodb_doublewrite to 0.
If system tablespace files (“ibdata files”) are
located on Fusion-io devices that support atomic writes,
doublewrite buffering is automatically disabled and Fusion-io
atomic writes are used for all data files. Because the doublewrite
buffer setting is global, doublewrite buffering is also disabled
for data files residing on non-Fusion-io hardware. This feature is
only supported on Fusion-io hardware and is only enabled for
Fusion-io NVMFS on Linux. To take full advantage of this feature,
an innodb_flush_method setting of
O_DIRECT is recommended.