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 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,
innodb_flush_method setting of
O_DIRECT is recommended.