From 3b682df3260aa8e020201e4b6c5cbc31fe8ecb8e Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Tue, 6 Mar 2012 09:13:00 +0200
Subject: [PATCH] Simplify the way changes to full_page_writes are logged.

It's harmless to do full page writes even when not strictly necessary, so
when turning full_page_writes on, we can set the global flag first, and then
call XLogInsert. Likewise, when turning it off, we can write the WAL record
first, and then clear the flag. This way XLogInsert doesn't need any special
handling of the XLOG_FPW_CHANGE record type. XLogInsert is complicated
enough already, so anything we can keep away from there is a good thing.

Actually I don't think the atomicity of the shared memory flag matters,
anyway, because we only write the XLOG_FPW_CHANGE at the end of recovery,
when there are no concurrent WAL insertions going on. But might as well make
it safe, in case we allow changing full_page_writes on the fly in the
future.
---
 src/backend/access/transam/xlog.c | 60 +++++++++++++------------------
 1 file changed, 25 insertions(+), 35 deletions(-)

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 266c0decac..eb7932e90a 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -731,8 +731,7 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
 	unsigned	i;
 	bool		updrqst;
 	bool		doPageWrites;
-	bool		isLogSwitch = false;
-	bool		fpwChange = false;
+	bool		isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
 	uint8		info_orig = info;
 
 	/* cross-check on whether we should be here or not */
@@ -746,30 +745,11 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
 	TRACE_POSTGRESQL_XLOG_INSERT(rmid, info);
 
 	/*
-	 * Handle special cases/records.
+	 * In bootstrap mode, we don't actually log anything but XLOG resources;
+	 * return a phony record pointer.
 	 */
-	if (rmid == RM_XLOG_ID)
+	if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
 	{
-		switch (info)
-		{
-			case XLOG_SWITCH:
-				isLogSwitch = true;
-				break;
-
-			case XLOG_FPW_CHANGE:
-				fpwChange = true;
-				break;
-
-			default:
-				break;
-		}
-	}
-	else if (IsBootstrapProcessingMode())
-	{
-		/*
-		 * In bootstrap mode, we don't actually log anything but XLOG resources;
-		 * return a phony record pointer.
-		 */
 		RecPtr.xlogid = 0;
 		RecPtr.xrecoff = SizeOfXLogLongPHD;		/* start of 1st chkpt record */
 		return RecPtr;
@@ -1232,15 +1212,6 @@ begin:;
 		WriteRqst = XLogCtl->xlblocks[curridx];
 	}
 
-	/*
-	 * If the record is an XLOG_FPW_CHANGE, we update full_page_writes
-	 * in shared memory before releasing WALInsertLock. This ensures that
-	 * an XLOG_FPW_CHANGE record precedes any WAL record affected
-	 * by this change of full_page_writes.
-	 */
-	if (fpwChange)
-		Insert->fullPageWrites = fullPageWrites;
-
 	LWLockRelease(WALInsertLock);
 
 	if (updrqst)
@@ -8517,6 +8488,22 @@ UpdateFullPageWrites(void)
 	if (fullPageWrites == Insert->fullPageWrites)
 		return;
 
+	START_CRIT_SECTION();
+
+	/*
+	 * It's always safe to take full page images, even when not strictly
+	 * required, but not the other round. So if we're setting full_page_writes
+	 * to true, first set it true and then write the WAL record. If we're
+	 * setting it to false, first write the WAL record and then set the
+	 * global flag.
+	 */
+	if (fullPageWrites)
+	{
+		LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
+		Insert->fullPageWrites = true;
+		LWLockRelease(WALInsertLock);
+	}
+
 	/*
 	 * Write an XLOG_FPW_CHANGE record. This allows us to keep
 	 * track of full_page_writes during archive recovery, if required.
@@ -8532,12 +8519,15 @@ UpdateFullPageWrites(void)
 
 		XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE, &rdata);
 	}
-	else
+
+
+	if (!fullPageWrites)
 	{
 		LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
-		Insert->fullPageWrites = fullPageWrites;
+		Insert->fullPageWrites = false;
 		LWLockRelease(WALInsertLock);
 	}
+	END_CRIT_SECTION();
 }
 
 /*