InnoDB Doublewrite Buffer
The InnoDB doublewrite buffer was implemented to recover from half-written pages. This can happen when there's a power failure while InnoDB is writing a page to disk. On reading that page, InnoDB can discover the corruption from the mismatch of the page checksum. However, in order to recover, an intact copy of the page would be needed.
The double write buffer provides such a copy.
Whenever InnoDB flushes a page to disk, it is first written to the double write buffer. Only when the buffer is safely flushed to disk will InnoDB write the page to the final destination. When recovering, InnoDB scans the double write buffer and for each valid page in the buffer checks if the page in the data file is valid too.
Doublewrite Buffer Settings
To turn off the doublewrite buffer, set the innodb_doublewrite system variable to
0. This is safe on filesystems that write pages atomically - that is, a page write fully succeeds or fails. But with other filesystems, it is not recommended for production systems. An alternative option is atomic writes. See atomic write support for more details.