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

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

2 files changed, 25 insertions, 26 deletions

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index ce84dac0c40..475f5ed8611 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -2217,28 +2217,24 @@ SetMultiXactIdLimit(MultiXactId oldest_datminmxid, Oid oldest_datoid,
 		multiWrapLimit += FirstMultiXactId;
 
 	/*
-	 * We'll refuse to continue assigning MultiXactIds once we get within 100
-	 * multi of data loss.
-	 *
-	 * Note: This differs from the magic number used in
-	 * SetTransactionIdLimit() since vacuum itself will never generate new
-	 * multis.  XXX actually it does, if it needs to freeze old multis.
+	 * We'll refuse to continue assigning MultiXactIds once we get within 3M
+	 * multi of data loss.  See SetTransactionIdLimit.
 	 */
-	multiStopLimit = multiWrapLimit - 100;
+	multiStopLimit = multiWrapLimit - 3000000;
 	if (multiStopLimit < FirstMultiXactId)
 		multiStopLimit -= FirstMultiXactId;
 
 	/*
-	 * We'll start complaining loudly when we get within 10M multis of the
-	 * stop point.   This is kind of arbitrary, but if you let your gas gauge
-	 * get down to 1% of full, would you be looking for the next gas station?
-	 * We need to be fairly liberal about this number because there are lots
-	 * of scenarios where most transactions are done by automatic clients that
-	 * won't pay attention to warnings. (No, we're not gonna make this
+	 * We'll start complaining loudly when we get within 40M multis of data
+	 * loss.  This is kind of arbitrary, but if you let your gas gauge get
+	 * down to 2% of full, would you be looking for the next gas station?  We
+	 * need to be fairly liberal about this number because there are lots of
+	 * scenarios where most transactions are done by automatic clients that
+	 * won't pay attention to warnings.  (No, we're not gonna make this
 	 * configurable.  If you know enough to configure it, you know enough to
 	 * not get in this kind of trouble in the first place.)
 	 */
-	multiWarnLimit = multiStopLimit - 10000000;
+	multiWarnLimit = multiWrapLimit - 40000000;
 	if (multiWarnLimit < FirstMultiXactId)
 		multiWarnLimit -= FirstMultiXactId;
 
diff --git a/src/backend/access/transam/varsup.c b/src/backend/access/transam/varsup.c
index e14b53bf9e3..0142bc70f6a 100644
--- a/src/backend/access/transam/varsup.c
+++ b/src/backend/access/transam/varsup.c
@@ -350,27 +350,30 @@ SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
 
 	/*
 	 * We'll refuse to continue assigning XIDs in interactive mode once we get
-	 * within 1M transactions of data loss.  This leaves lots of room for the
+	 * within 3M transactions of data loss.  This leaves lots of room for the
 	 * DBA to fool around fixing things in a standalone backend, while not
 	 * being significant compared to total XID space. (Note that since
 	 * vacuuming requires one transaction per table cleaned, we had better be
-	 * sure there's lots of XIDs left...)
+	 * sure there's lots of XIDs left...)  Also, at default BLCKSZ, this
+	 * leaves two completely-idle segments.  In the event of edge-case bugs
+	 * involving page or segment arithmetic, idle segments render the bugs
+	 * unreachable outside of single-user mode.
 	 */
-	xidStopLimit = xidWrapLimit - 1000000;
+	xidStopLimit = xidWrapLimit - 3000000;
 	if (xidStopLimit < FirstNormalTransactionId)
 		xidStopLimit -= FirstNormalTransactionId;
 
 	/*
-	 * We'll start complaining loudly when we get within 10M transactions of
-	 * the stop point.  This is kind of arbitrary, but if you let your gas
-	 * gauge get down to 1% of full, would you be looking for the next gas
-	 * station?  We need to be fairly liberal about this number because there
-	 * are lots of scenarios where most transactions are done by automatic
-	 * clients that won't pay attention to warnings. (No, we're not gonna make
-	 * this configurable.  If you know enough to configure it, you know enough
-	 * to not get in this kind of trouble in the first place.)
+	 * We'll start complaining loudly when we get within 40M transactions of
+	 * data loss.  This is kind of arbitrary, but if you let your gas gauge
+	 * get down to 2% of full, would you be looking for the next gas station?
+	 * We need to be fairly liberal about this number because there are lots
+	 * of scenarios where most transactions are done by automatic clients that
+	 * won't pay attention to warnings.  (No, we're not gonna make this
+	 * configurable.  If you know enough to configure it, you know enough to
+	 * not get in this kind of trouble in the first place.)
 	 */
-	xidWarnLimit = xidStopLimit - 10000000;
+	xidWarnLimit = xidWrapLimit - 40000000;
 	if (xidWarnLimit < FirstNormalTransactionId)
 		xidWarnLimit -= FirstNormalTransactionId;