The Doublewrite Buffer provides a copy of a page needed to recover from a page corruption. This kind of corruption can happen when there is a power failure while InnoDB is writing a page to disk. InnoDB can track down the corruption from the mismatch of the page checksum while reading that page.
InnoDB writes a page to the doublewrite buffer first whenever it flushes page to disk. InnoDB will write the page to the final destination only when the buffer is safely flushed to disk. When recovering, InnoDB scans the double write buffer and for each valid page in the buffer, and checks if the page in the data file is valid too.
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