Re: pg_upgrade can result in early wraparound on databases with hightransaction load

On Tue, May 21, 2019 at 03:23:00PM -0700, Peter Geoghegan wrote:
> On Mon, May 20, 2019 at 3:10 AM Jason Harvey <jason@reddit.com> wrote:
> > This week I upgraded one of my large(2.8TB), high-volume databases from 9 to 11. The upgrade itself went fine.
Abouttwo days later, we unexpectedly hit transaction ID wraparound. What was perplexing about this was that the age of
ouroldest `datfrozenxid` was only 1.2 billion - far away from where I'd expect a wraparound. Curiously, the wraparound
errorreferred to a mysterious database of `OID 0`:
 
> >
> > UPDATE ERROR:  database is not accepting commands to avoid wraparound data loss in database with OID 0

That's bad.

> > We were able to recover after a few hours by greatly speeding up our vacuum on our largest table.

For what it's worth, a quicker workaround is to VACUUM FREEZE any database,
however small.  That forces a vac_truncate_clog(), which recomputes the wrap
point from pg_database.datfrozenxid values.  This demonstrates the workaround:

--- a/src/bin/pg_upgrade/test.sh
+++ b/src/bin/pg_upgrade/test.sh
@@ -248,7 +248,10 @@ case $testhost in
 esac
 
 pg_dumpall --no-sync -f "$temp_root"/dump2.sql || pg_dumpall2_status=$?
+pg_controldata "${PGDATA}"
+vacuumdb -F template1
 pg_ctl -m fast stop
+pg_controldata "${PGDATA}"
 
 if [ -n "$pg_dumpall2_status" ]; then
     echo "pg_dumpall of post-upgrade database cluster failed"

> > In a followup investigation I uncovered the reason we hit the wraparound so early, and also the cause of the
mysteriousOID 0 message. When pg_upgrade executes, it calls pg_resetwal to set the next transaction ID. Within
pg_resetwalis the following code:
https://github.com/postgres/postgres/blob/6cd404b344f7e27f4d64555bb133f18a758fe851/src/bin/pg_resetwal/pg_resetwal.c#L440-L450

pg_upgrade should set oldestXID to the same value as the source cluster or set
it like vac_truncate_clog() would set it.  Today's scheme is usually too
pessimistic, but it can be too optimistic if the source cluster was on the
bring of wrap.  Thanks for the report.