pg_upgrade: document use of file system and COW snapshots

diff --git a/doc/src/sgml/pgupgrade.sgml b/doc/src/sgml/pgupgrade.sgml
index 3d529b29c1242a37ddac0e39e36d2caef45f0dab..bb3b6a0a93357010a42e1cbc98863d2cde07e465 100644 (file)
--- a/doc/src/sgml/pgupgrade.sgml
+++ b/doc/src/sgml/pgupgrade.sgml
@@ -569,7 +569,10 @@ psql --username postgres --file script.sql postgres
    the old server and run rsync again to update the copy with any
    changes to make it consistent.  You might want to exclude some
    files, e.g. postmaster.pid, as documented in 
-   linkend="backup-lowlevel-base-backup">.
+   linkend="backup-lowlevel-base-backup">.  If your file system supports
+   file system snapshots or copy-on-write file copying, you can use that
+   to make a backup of the old cluster, though the snapshot and copies
+   must be created simultaneously or while the database server is down.