diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 1424cab..ddedade 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -204,6 +204,7 @@ typedef struct MultiXactStateData
 	 */
 	MultiXactId oldestMultiXactId;
 	Oid			oldestMultiXactDB;
+	MultiXactOffset oldestOffset;
 
 	/*
 	 * This is what the previous checkpoint stored as the truncate position.
@@ -954,14 +955,16 @@ GetNewMultiXactId(int nmembers, MultiXactOffset *offset)
 	 * against catastrophic data loss due to multixact wraparound.  The basic
 	 * rules are:
 	 *
-	 * If we're past multiVacLimit, start trying to force autovacuum cycles.
+	 * If we're past multiVacLimit or the safe threshold for member storage space,
+	 * start trying to force autovacuum cycles.
 	 * If we're past multiWarnLimit, start issuing warnings.
 	 * If we're past multiStopLimit, refuse to create new MultiXactIds.
 	 *
 	 * Note these are pretty much the same protections in GetNewTransactionId.
 	 *----------
 	 */
-	if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit))
+	if (!MultiXactIdPrecedes(result, MultiXactState->multiVacLimit) ||
+		(MultiXactState->nextOffset - MultiXactState->oldestOffset > MULTIXACT_MEMBER_SAFE_THRESHOLD))
 	{
 		/*
 		 * For safety's sake, we release MultiXactGenLock while sending
@@ -2482,6 +2485,7 @@ static void
 DetermineSafeOldestOffset(MultiXactId oldestMXact)
 {
 	MultiXactOffset oldestOffset;
+	MultiXactOffset startOfOldestSegment;
 
 	/*
 	 * Can't do this while initdb'ing or in the startup process while
@@ -2505,8 +2509,9 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
 
 	LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
 	/* always leave one segment before the wraparound point */
-	MultiXactState->offsetStopLimit = oldestOffset -
+	MultiXactState->offsetStopLimit = startOfOldestSegment -
 		(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);
+	MultiXactState->oldestOffset = oldestOffset;
 	LWLockRelease(MultiXactGenLock);
 }
 
@@ -2599,9 +2604,9 @@ ReadMultiXactCounts(uint32 *multixacts, MultiXactOffset *members)
 	nextOffset = MultiXactState->nextOffset;
 	oldestMultiXactId = MultiXactState->oldestMultiXactId;
 	nextMultiXactId = MultiXactState->nextMXact;
+	oldestOffset = MultiXactState->oldestOffset;
 	LWLockRelease(MultiXactGenLock);
 
-	oldestOffset = find_multixact_start(oldestMultiXactId);
 	*members = nextOffset - oldestOffset;
 	*multixacts = nextMultiXactId - oldestMultiXactId;
 }