MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

15.6.4 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 unexpected mysqld process exit 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).

Prior to MySQL 8.0.20, the doublewrite buffer storage area is located in the InnoDB system tablespace. As of MySQL 8.0.20, the doublewrite buffer storage area is located in doublewrite files.

The following variables are provided for doublewrite buffer configuration:

As of MySQL 8.0.23, InnoDB automatically encrypts doublewrite file pages that belong to encrypted tablespaces (see Section 15.13, “InnoDB Data-at-Rest Encryption”). Likewise, doublewrite file pages belonging page-compressed tablespaces are compressed. As a result, doublewrite files can contain different page types including unencrypted and uncompressed pages, encrypted pages, compressed pages, and pages that are both encrypted and compressed.