doc: Mention clock synchronization recommendation for hot_standby_feedback

petere · petere · commit 0fcf02ad4568 · 2025-03-31T16:54:50.000+02:00
hot_standby_feedback mechanics assume that clocks are synchronized, but it was not clear from documentation. Author: Jakub Wartak <jakub.wartak@enterprisedb.com> Reviewed-by: Euler Taveira <euler@eulerto.com> Reviewed-by: Amit Kapila <amit.kapila16@gmail.com> Reviewed-by: vignesh C <vignesh21@gmail.com> Discussion: https://postgr.es/m/CAKZiRmwBcALLrDgCyEhHP1enUxtPMjyNM_d1A2Lng3_6Rf4Qfw%40mail.gmail.com
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
@@ -5068,6 +5068,13 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         until it eventually reaches the primary.  Standbys make no other use
         of feedback they receive other than to pass upstream.
        </para>
+       <para>
+        Note that if the clock on standby is moved ahead or backward, the
+        feedback message might not be sent at the required interval.  In
+        extreme cases, this can lead to a prolonged risk of not removing dead
+        rows on the primary for extended periods, as the feedback mechanism is
+        based on timestamps.
+       </para>
       </listitem>
      </varlistentry>
 
