14.6.5 Doublewrite Buffer
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 mysqld process crash 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.