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Change XID and mxact limits to warn at 40M and stop at 3M.

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We have edge-case bugs when assigning values in the last few dozen pages
before the wrap limit.  We may introduce similar bugs in the future.  At
default BLCKSZ, this makes such bugs unreachable outside of single-user
mode.  Also, when VACUUM began to consume mxacts, multiStopLimit did not
change to compensate.

pg_upgrade may fail on a cluster that was already printing "must be
vacuumed" warnings.  Follow the warning's instructions to clear the
warning, then run pg_upgrade again.  One can still, peacefully consume
98% of XIDs or mxacts, so DBAs need not change routine VACUUM settings.

Discussion: https://postgr.es/m/20200621083513.GA3074645@rfd.leadboat.com
This commit is contained in:
Noah Misch
2020-08-01 15:31:01 -07:00
parent 9f9682783b
commit cd5e82256d
3 changed files with 29 additions and 30 deletions
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8
doc/src/sgml/maintenance.sgml
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@ -608,10 +608,10 @@ SELECT datname, age(datfrozenxid) FROM pg_database;
<para>
If for some reason autovacuum fails to clear old XIDs from a table, the
system will begin to emit warning messages like this when the database's
oldest XIDs reach eleven million transactions from the wraparound point:
oldest XIDs reach forty million transactions from the wraparound point:
<programlisting>
WARNING: database "mydb" must be vacuumed within 10985967 transactions
WARNING: database "mydb" must be vacuumed within 39985967 transactions
HINT: To avoid a database shutdown, execute a database-wide VACUUM in that database.
</programlisting>
@ -621,7 +621,7 @@ HINT: To avoid a database shutdown, execute a database-wide VACUUM in that data
be able to advance the database's <structfield>datfrozenxid</structfield>.)
If these warnings are
ignored, the system will shut down and refuse to start any new
transactions once there are fewer than 1 million transactions left
transactions once there are fewer than three million transactions left
until wraparound:
<programlisting>
@ -629,7 +629,7 @@ ERROR: database is not accepting commands to avoid wraparound data loss in data
HINT: Stop the postmaster and vacuum that database in single-user mode.
</programlisting>
The 1-million-transaction safety margin exists to let the
The three-million-transaction safety margin exists to let the
administrator recover without data loss, by manually executing the
required <command>VACUUM</command> commands. However, since the system will not
execute commands once it has gone into the safety shutdown mode,