From: Simon Riggs <simon@2ndQuadrant.com>
Date: Thu, 7 Jun 2012 18:24:47 +0000 (+0100)
Subject: Wake WALSender to reduce data loss at failover for async commit.
X-Git-Tag: REL9_1_5~61
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=16222f32ed56d3ebc4136133662d932299188955;p=postgresql

Wake WALSender to reduce data loss at failover for async commit.
WALSender now woken up after each background flush by WALwriter, avoiding
multi-second replication delay for an all-async commit workload.
Replication delay reduced from 7s with default settings to 200ms, allowing
significantly reduced data loss at failover.

Andres Freund and Simon Riggs
---

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index f0db58fb6e..8988993746 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2168,6 +2168,7 @@ XLogBackgroundFlush(void)
 {
 	XLogRecPtr	WriteRqstPtr;
 	bool		flexible = true;
+	bool		wrote_something = false;
 
 	/* XLOG doesn't need flushing during recovery */
 	if (RecoveryInProgress())
@@ -2236,10 +2237,18 @@ XLogBackgroundFlush(void)
 		WriteRqst.Write = WriteRqstPtr;
 		WriteRqst.Flush = WriteRqstPtr;
 		XLogWrite(WriteRqst, flexible, false);
+		wrote_something = true;
 	}
 	LWLockRelease(WALWriteLock);
 
 	END_CRIT_SECTION();
+
+	/*
+	 * If we wrote something then we have something to send to standbys also,
+	 * otherwise the replication delay become around 7s with just async commit.
+	 */
+	if (wrote_something)
+		WalSndWakeup();
 }
 
 /*