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Multixact member files are subject to early wraparound overflow and removal: if the average multixact size is above a certain threshold (see note below) the protections against offset overflow are not enough: during multixact truncation at checkpoint time, some pg_multixact/members files would be removed because the server considers them to be old and not needed anymore. This leads to loss of files that are critical to interpret existing tuples's Xmax values. To protect against this, since we don't have enough info in pg_control and we can't modify it in old branches, we maintain shared memory state about the oldest value that we need to keep; we use this during new multixact creation to abort if an old still-needed file would get overwritten. This value is kept up to date by checkpoints, which makes it not completely accurate but should be good enough. We start emitting warnings sometime earlier, so that the eventual multixact-shutdown doesn't take DBAs completely by surprise (more precisely: once 20 members SLRU segments are remaining before shutdown.) On troublesome average multixact size: The threshold size depends on the multixact freeze parameters. The oldest age is related to the greater of multixact_freeze_table_age and multixact_freeze_min_age: anything older than that should be removed promptly by autovacuum. If autovacuum is keeping up with multixact freezing, the troublesome multixact average size is (2^32-1) / Max(freeze table age, freeze min age) or around 28 members per multixact. Having an average multixact size larger than that will eventually cause new multixact data to overwrite the data area for older multixacts. (If autovacuum is not able to keep up, or there are errors in vacuuming, the actual maximum is multixact_freeeze_max_age instead, at which point multixact generation is stopped completely. The default value for this limit is 400 million, which means that the multixact size that would cause trouble is about 10 members). Initial bug report by Timothy Garnett, bug #12990 Backpatch to 9.3, where the problem was introduced. Authors: Álvaro Herrera, Thomas Munro Reviews: Thomas Munro, Amit Kapila, Robert Haas, Kevin Grittner |
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README
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.