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Peter Geoghegan 1e55e7d175 Add wraparound failsafe to VACUUM. ... Add a failsafe mechanism that is triggered by VACUUM when it notices that the table's relfrozenxid and/or relminmxid are dangerously far in the past. VACUUM checks the age of the table dynamically, at regular intervals. When the failsafe triggers, VACUUM takes extraordinary measures to finish as quickly as possible so that relfrozenxid and/or relminmxid can be advanced. VACUUM will stop applying any cost-based delay that may be in effect. VACUUM will also bypass any further index vacuuming and heap vacuuming -- it only completes whatever remaining pruning and freezing is required. Bypassing index/heap vacuuming is enabled by commit 8523492d, which made it possible to dynamically trigger the mechanism already used within VACUUM when it is run with INDEX_CLEANUP off. It is expected that the failsafe will almost always trigger within an autovacuum to prevent wraparound, long after the autovacuum began. However, the failsafe mechanism can trigger in any VACUUM operation. Even in a non-aggressive VACUUM, where we're likely to not advance relfrozenxid, it still seems like a good idea to finish off remaining pruning and freezing. An aggressive/anti-wraparound VACUUM will be launched immediately afterwards. Note that the anti-wraparound VACUUM that follows will itself trigger the failsafe, usually before it even begins its first (and only) pass over the heap. The failsafe is controlled by two new GUCs: vacuum_failsafe_age, and vacuum_multixact_failsafe_age. There are no equivalent reloptions, since that isn't expected to be useful. The GUCs have rather high defaults (both default to 1.6 billion), and are expected to generally only be used to make the failsafe trigger sooner/more frequently. Author: Masahiko Sawada <sawada.mshk@gmail.com> Author: Peter Geoghegan <pg@bowt.ie> Discussion: https://postgr.es/m/CAD21AoD0SkE11fMw4jD4RENAwBMcw1wasVnwpJVw3tVqPOQgAw@mail.gmail.com Discussion: https://postgr.es/m/CAH2-WzmgH3ySGYeC-m-eOBsa2=sDwa292-CFghV4rESYo39FsQ@mail.gmail.com		4 years ago
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src	Add wraparound failsafe to VACUUM.	4 years ago
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