Expand comment on how pg_upgrade is turning off autovacuum.

diff --git a/contrib/pg_upgrade/server.c b/contrib/pg_upgrade/server.c
index 4d5f37301b46ba35525eb0280e4e2717f7e739c1..9331667b8b7e29fc0ceb7089cad815c40c399f48 100644 (file)
--- a/contrib/pg_upgrade/server.c
+++ b/contrib/pg_upgrade/server.c
@@ -172,7 +172,9 @@ start_postmaster(ClusterInfo *cluster, bool quiet)
     *
     * Using autovacuum=off disables cleanup vacuum and analyze, but
     * freeze vacuums can still happen, so we set
-    * autovacuum_freeze_max_age high too.
+    * autovacuum_freeze_max_age very high.  We assume all datfrozenxid and
+    * relfrozen values are less than a gap of 2000000000 from the current
+    * xid counter, so autovacuum will not touch them.
     */ 
    snprintf(cmd, sizeof(cmd),
             SYSTEMQUOTE "\"%s/pg_ctl\" -l \"%s\" -D \"%s\" "