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When I (rhaas) wrote the WAL summarizer code, I incorrectly believed that XLOG_SMGR_TRUNCATE truncates all forks to the same length. In fact, what other parts of the code do is compute the truncation length for the FSM and VM forks from the truncation length used for the main fork. But, because I was confused, I coded the WAL summarizer to set the limit block for the VM fork to the same value as for the main fork. (Incremental backup always copies FSM forks in full, so there is no similar issue in that case.) Doing that doesn't directly cause any data corruption, as far as I can see. However, it does create a serious risk of consuming a large amount of extra disk space, because pg_combinebackup's reconstruct.c believes that the reconstructed file should always be at least as long as the limit block value. We might want to be smarter about that at some point in the future, because it's always safe to omit all-zeroes blocks at the end of the last segment of a relation, and doing so could save disk space, but the current algorithm will rarely waste enough disk space to worry about unless we believe that a relation has been truncated to a length much longer than its actual length on disk, which is exactly what happens as a result of the problem mentioned in the previous paragraph. To fix, create a new visibilitymap helper function and use it to include the right limit block in the summary files. Incremental backups taken with existing summary files will still have this issue, but this should improve the situation going forward. Diagnosed-by: Oleg Tkachenko <oatkachenko@gmail.com> Diagnosed-by: Amul Sul <sulamul@gmail.com> Discussion: http://postgr.es/m/CAAJ_b97PqG89hvPNJ8cGwmk94gJ9KOf_pLsowUyQGZgJY32o9g@mail.gmail.com Discussion: http://postgr.es/m/6897DAF7-B699-41BF-A6FB-B818FCFFD585%40gmail.com Backpatch-through: 17 |
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