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MySQL 5.5 Reference Manual
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14.7.7 Doublewrite Buffer

The doublewrite buffer is a storage area located in the system tablespace where 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 write, 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 fsync() call to the operating system.

The doublewrite buffer is enabled by default. To disable the doublewrite buffer, set innodb_doublewrite to 0.


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