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The purpose of the InnoDB doublewrite buffer is to make InnoDB tolerant against cases where the server was killed in the middle of a page write. (In Linux, killing a process may interrupt a write system call, typically on a 4096-byte boundary.) There may exist multiple copies of a page number in the doublewrite buffer. Recovery should choose the latest valid copy of the page. By design, the FIL_PAGE_LSN must not precede the latest checkpoint LSN nor be later than the end of the recovered log. For page_compressed and encrypted pages, we were missing proper consistency checks. In the 10.4 data set generated for in MDEV-23231, the data file contained a valid page_compressed page, and an identical copy of that page was also present in the doublewrite buffer. But, recovery would incorrectly consider the page invalid and restore an uncompressed copy of the same page that had been written before the log checkpoint. (In fact, no redo log was to be applied to that page.) buf_dblwr_process(): Validate the FIL_PAGE_LSN in the doublewrite buffer pages, and always skip page 0, because those pages should have been recovered by Datafile::restore_from_doublewrite() if necessary. Datafile::restore_from_doublewrite(): Choose the latest applicable page from the doublewrite buffer. recv_dblwr_t::find_page(): Also validate encrypted or page_compressed pages. recv_dblwr_t::validate_page(): New function to validate a page, either a copy in a data file or in the doublewrite buffer. Also validate encrypted or page_compressed pages. This is joint work with Thirunarayanan Balathandayuthapani. |
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