This eliminates MultiXactOffset wraparound and the 2^32 limit on the total number of multixid members. Multixids are still limited to 2^31, but this is a nice improvement because 'members' can grow much faster than the number of multixids. On such systems, you can now run longer before hitting hard limits or triggering anti-wraparound vacuums. Not having to deal with MultiXactOffset wraparound also simplifies the code and removes some gnarly corner cases. We no longer need to perform emergency anti-wraparound freezing because of running out of 'members' space, so the offset stop limit is gone. But you might still not want 'members' to consume huge amounts of disk space. For that reason, I kept the logic for lowering vacuum's multixid freezing cutoff if a large amount of 'members' space is used. The thresholds for that are roughly the same as the "safe" and "danger" thresholds used before, 2 billion transactions and 4 billion transactions. This keeps the behavior for the freeze cutoff roughly the same as before. It might make sense to make this smarter or configurable, now that the threshold is only needed to manage disk usage, but that's left for the future. Add code to pg_upgrade to convert multitransactions from the old to the new format, rewriting the pg_multixact SLRU files. Because pg_upgrade now rewrites the files, we can get rid of some hacks we had put in place to deal with old bugs and upgraded clusters. Bump catalog version for the pg_multixact/offsets format change. Author: Maxim Orlov <orlovmg@gmail.com> Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com> Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com> Discussion: https://www.postgresql.org/message-id/CACG%3DezaWg7_nt-8ey4aKv2w9LcuLthHknwCawmBgEeTnJrJTcw@mail.gmail.com