Document restrictions regarding incremental backups and standbys.

If you try to take an incremental backup on a standby and there hasn't
been much system activity, it might fail. Document why this happens.
Also add a hint to the error message you get, to make it more likely
that users will understand what has gone wrong.

Laurenz Albe and Robert Haas

Discussion: https://postgr.es/m/5468641ad821dad7aa3b2d65bf843146443a1b68.camel@cybertec.at

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/src/sgml/backup.sgml b/doc/src/sgml/backup.sgml
index 91da3c26ba0..e4e4c56cf14 100644
--- a/doc/src/sgml/backup.sgml
+++ b/doc/src/sgml/backup.sgml
@@ -925,6 +925,17 @@ test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/0
     to manage. For a large database all of which is heavily modified,
     incremental backups won't be much smaller than full backups.
    </para>
+
+   <para>
+    An incremental backup is only possible if replay would begin from a later
+    checkpoint than for the previous backup upon which it depends.  If you
+    take the incremental backup on the primary, this condition is always
+    satisfied, because each backup triggers a new checkpoint.  On a standby,
+    replay begins from the most recent restartpoint.  Therefore, an
+    incremental backup of a standby server can fail if there has been very
+    little activity since the previous backup, since no new restartpoint might
+    have been created.
+   </para>
   </sect2>
 
   <sect2 id="backup-lowlevel-base-backup">