Arrange to call AbsorbFsyncRequests every so often while performing a

checkpoint in the bgwriter.  This forestalls overflow of the fsync request
queue, which is not fatal but causes considerable performance degradation
when it occurs (because backends then have to do their own fsyncs).  Per
patch from Itagaki Takahiro, modified a little bit by me.

src/backend/storage/buffer/bufmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.202 2006/01/06 00:04:20 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.203 2006/03/03 00:02:01 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -42,6 +42,7 @@
 
 #include "lib/stringinfo.h"
 #include "miscadmin.h"
+#include "postmaster/bgwriter.h"
 #include "storage/buf_internals.h"
 #include "storage/bufmgr.h"
 #include "storage/bufpage.h"
@@ -61,6 +62,9 @@
 #define LocalBufHdrGetBlock(bufHdr) \
 	LocalBufferBlockPointers[-((bufHdr)->buf_id + 2)]
 
+/* interval for calling AbsorbFsyncRequests in BufferSync */
+#define WRITES_PER_ABSORB		1000
+
 
 /* GUC variables */
 bool		zero_damaged_pages = false;
@@ -892,6 +896,7 @@ BufferSync(void)
 {
 	int			buf_id;
 	int			num_to_scan;
+	int			absorb_counter;
 
 	/*
 	 * Find out where to start the circular scan.
@@ -905,9 +910,23 @@ BufferSync(void)
 	 * Loop over all buffers.
 	 */
 	num_to_scan = NBuffers;
+	absorb_counter = WRITES_PER_ABSORB;
 	while (num_to_scan-- > 0)
 	{
-		(void) SyncOneBuffer(buf_id, false);
+		if (SyncOneBuffer(buf_id, false))
+		{
+			/*
+			 * If in bgwriter, absorb pending fsync requests after each
+			 * WRITES_PER_ABSORB write operations, to prevent overflow of
+			 * the fsync request queue.  If not in bgwriter process, this is
+			 * a no-op.
+			 */
+			if (--absorb_counter <= 0)
+			{
+				AbsorbFsyncRequests();
+				absorb_counter = WRITES_PER_ABSORB;
+			}
+		}
 		if (++buf_id >= NBuffers)
 			buf_id = 0;
 	}

src/backend/storage/smgr/md.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.118 2005/10/15 02:49:26 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/smgr/md.c,v 1.119 2006/03/03 00:02:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -28,6 +28,9 @@
 #include "utils/memutils.h"
 
 
+/* interval for calling AbsorbFsyncRequests in mdsync */
+#define FSYNCS_PER_ABSORB		10
+
 /*
  *	The magnetic disk storage manager keeps track of open file
  *	descriptors in its own descriptor pool.  This is done to make it
@@ -702,6 +705,7 @@ mdsync(void)
 {
 	HASH_SEQ_STATUS hstat;
 	PendingOperationEntry *entry;
+	int			absorb_counter;
 
 	if (!pendingOpsTable)
 		return false;
@@ -714,6 +718,7 @@ mdsync(void)
 	 */
 	AbsorbFsyncRequests();
 
+	absorb_counter = FSYNCS_PER_ABSORB;
 	hash_seq_init(&hstat, pendingOpsTable);
 	while ((entry = (PendingOperationEntry *) hash_seq_search(&hstat)) != NULL)
 	{
@@ -727,6 +732,19 @@ mdsync(void)
 			SMgrRelation reln;
 			MdfdVec    *seg;
 
+			/*
+			 * If in bgwriter, absorb pending requests every so often to
+			 * prevent overflow of the fsync request queue.  The hashtable
+			 * code does not specify whether entries added by this will be
+			 * visited by our search, but we don't really care: it's OK if
+			 * we do, and OK if we don't.
+			 */
+			if (--absorb_counter <= 0)
+			{
+				AbsorbFsyncRequests();
+				absorb_counter = FSYNCS_PER_ABSORB;
+			}
+
 			/*
 			 * Find or create an smgr hash entry for this relation. This may
 			 * seem a bit unclean -- md calling smgr?  But it's really the