From: Simon Riggs <simon@2ndQuadrant.com>
Date: Tue, 22 Mar 2011 16:23:54 +0000 (+0000)
Subject: Update docs to say you need fsync to make sync rep work fast.
X-Git-Tag: REL9_1_ALPHA5~37
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=6e8e7cc580665ddd43c8ca2acc6d60f345570a57;p=postgresql

Update docs to say you need fsync to make sync rep work fast.
---

diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index a7e2474189..40b8b7393f 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -947,7 +947,7 @@ synchronous_replication = on
    <para>
     After a commit record has been written to disk on the primary the
     WAL record is then sent to the standby. The standby sends reply
-    messages each time a new batch of WAL data is received, unless
+    messages each time a new batch of WAL data is written to disk, unless
     <varname>wal_receiver_status_interval</> is set to zero on the standby.
     If the standby is the first matching standby, as specified in
     <varname>synchronous_standby_names</> on the primary, the reply
@@ -1013,6 +1013,12 @@ synchronous_replication = on
     the rate of generation of WAL data.
    </para>
 
+   <para>
+    Replies are only sent when WAL is written to disk, so setting
+    <varname>fsync</> to <literal>off</> on the standby will significantly
+    reduce performance of synchronous replication and should be avoided.
+   </para>
+
    </sect3>
 
    <sect3 id="synchronous-replication-ha">