commit 3916c54126ffade0baad4609467393d9a1b53e37
Author: Aidan Van Dyk <aidan@highrise.ca>
Date:   Fri Oct 31 12:35:24 2008 -0400

    WIP: Zero xlog tal on a forced switch
    
    If XLogWrite is called with xlog_switch, an XLog swithc has been force, either
    by a timeout based switch (archive_timeout), or an interactive force xlog
    switch (pg_switch_xlog/pg_stop_backup).  In those cases, we assume we can
    afford a little extra IO bandwidth to make xlogs so much more compressable

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8bc46da..a8d945d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -1548,6 +1548,30 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 			 */
 			if (finishing_seg || (xlog_switch && last_iteration))
 			{
+				/*
+				 * If we've had an xlog switch forced, then we want to zero
+				 * out the rest of the segment.  We zero it out here because at the
+				 * force switch time, IO bandwidth isn't a problem.
+				 *   -- AIDAN
+				 */
+				if (xlog_switch)
+				{
+					char buf[1024];
+					uint32 left = (XLogSegSize - openLogOff);
+					ereport(LOG,
+						(errmsg("ZEROING xlog file %u segment %u from %u - %u [%u bytes]",
+								openLogId, openLogSeg,
+								openLogOff, XLogSegSize, left)
+						 ));
+					memset(buf, 0, sizeof(buf));
+					while (left > 0)
+					{
+						size_t len = (left > sizeof(buf)) ? sizeof(buf) : left;
+						write(openLogFile, buf, len);
+						left -= len;
+					}
+				}
+
 				issue_xlog_fsync();
 				LogwrtResult.Flush = LogwrtResult.Write;		/* end of page */