Re: [HACKERS] WAL write of full pages

[Dennis Haney]

Bruce Momjian wrote: Our current WAL implementation writes copies of full pages to WAL before modifying the page on disk. This is done to prevent partial pages from being corrupted in case the operating system crashes during a page write. InnoDB uses a doublebuffer system instead. http://www.innodb.com/ibman.php#File.space.management quote: Starting from 3.23.40b, InnoDB uses a novel file flush technique called doublewrite. It adds safety to crash recovery after an operating system crash or a power outage, and improves performance on most Unix flavors by reducing the need for fsync() operations. Doublewrite means that InnoDB before writing pages to a data file first writes them to a contiguous tablespace area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed, InnoDB writes the pages to their proper positions in the data file. If the operating system crashes in the middle of a page write, InnoDB will in recovery find a good copy of the page from the doublewrite buffer. -- Dennis