diff doc/src/sgml/config.sgml
index 074afee..8ff21a9
*** a/doc/src/sgml/config.sgml
--- b/doc/src/sgml/config.sgml
*************** SET ENABLE_SEQSCAN TO OFF;
*** 1869,1888 ****
          When the commit data for a transaction is flushed to disk, any
          additional commits ready at that time are also flushed out.
          <varname>commit_delay</varname> adds a time delay, set in
!         microseconds, before a transaction attempts to
!         flush the WAL buffer out to disk.  A nonzero delay can allow more
!         transactions to be committed with only one flush operation, if
!         system load is high enough that additional transactions become
!         ready to commit within the given interval. But the delay is
!         just wasted if no other transactions become ready to
!         commit. Therefore, the delay is only performed if at least
!         <varname>commit_siblings</varname> other transactions are
!         active at the instant that a server process has written its
!         commit record.
          The default <varname>commit_delay</> is zero (no delay).
-         Since all pending commit data will be written at every flush
-         regardless of this setting, it is rare that adding delay
-         by increasing this parameter will actually improve performance.
         </para>
        </listitem>
       </varlistentry>
--- 1869,1888 ----
          When the commit data for a transaction is flushed to disk, any
          additional commits ready at that time are also flushed out.
          <varname>commit_delay</varname> adds a time delay, set in
!         microseconds, before a leading transaction participating in
!         group commit attempts to flush the WAL buffer out to disk.
!         This can add an additional latency of of up to
!         <varname>commit_delay</varname> microseconds for each transaction.
!         A nonzero delay can allow more transactions to be committed with
!         only one flush operation, if system load is high enough that
!         additional transactions become ready to commit within the
!         given interval. However, the delay is just wasted if no other
!         transactions become ready to commit. Therefore, the delay
!         is only performed if at least <varname>commit_siblings</varname>
!         other transactions are active immediately before the leader
!         backend participating in group commit proceeds with flushing
!         WAL.
          The default <varname>commit_delay</> is zero (no delay).
         </para>
        </listitem>
       </varlistentry>
diff doc/src/sgml/wal.sgml
index 0afb9d6..a98132d
*** a/doc/src/sgml/wal.sgml
--- b/doc/src/sgml/wal.sgml
***************
*** 376,384 ****
     <acronym>WAL</acronym> to disk, in the hope that a single flush
     executed by one such transaction can also serve other transactions
     committing at about the same time.  Setting <varname>commit_delay</varname>
!    can only help when there are many concurrently committing transactions,
!    and it is difficult to tune it to a value that actually helps rather
!    than hurt throughput.
    </para>

   </sect1>
--- 376,382 ----
     <acronym>WAL</acronym> to disk, in the hope that a single flush
     executed by one such transaction can also serve other transactions
     committing at about the same time.  Setting <varname>commit_delay</varname>
!    can only help when there are many concurrently committing transactions.
    </para>

   </sect1>
diff src/backend/access/transam/xact.c
index c71a10e..513172b
*** a/src/backend/access/transam/xact.c
--- b/src/backend/access/transam/xact.c
*************** bool		XactDeferrable;
*** 67,75 ****

  int			synchronous_commit = SYNCHRONOUS_COMMIT_ON;

- int			CommitDelay = 0;	/* precommit delay in microseconds */
- int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
-
  /*
   * MyXactAccessedTempRel is set when a temporary relation is accessed.
   * We don't allow PREPARE TRANSACTION in that case.  (This is global
--- 67,72 ----
*************** RecordTransactionCommit(void)
*** 1118,1139 ****
  	if ((wrote_xlog && synchronous_commit > SYNCHRONOUS_COMMIT_OFF) ||
  		forceSyncCommit || nrels > 0)
  	{
- 		/*
- 		 * Synchronous commit case:
- 		 *
- 		 * Sleep before flush! So we can flush more than one commit records
- 		 * per single fsync.  (The idea is some other backend may do the
- 		 * XLogFlush while we're sleeping.  This needs work still, because on
- 		 * most Unixen, the minimum select() delay is 10msec or more, which is
- 		 * way too long.)
- 		 *
- 		 * We do not sleep if enableFsync is not turned on, nor if there are
- 		 * fewer than CommitSiblings other backends with active transactions.
- 		 */
- 		if (CommitDelay > 0 && enableFsync &&
- 			MinimumActiveBackends(CommitSiblings))
- 			pg_usleep(CommitDelay);
-
  		XLogFlush(XactLastRecEnd);

  		/*
--- 1115,1120 ----
diff src/backend/access/transam/xlog.c
index d3650bd..3ef8d88
*** a/src/backend/access/transam/xlog.c
--- b/src/backend/access/transam/xlog.c
*************** bool		fullPageWrites = true;
*** 80,85 ****
--- 80,87 ----
  bool		log_checkpoints = false;
  int			sync_method = DEFAULT_SYNC_METHOD;
  int			wal_level = WAL_LEVEL_MINIMAL;
+ int			CommitDelay = 0;	/* precommit delay in microseconds */
+ int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */

  #ifdef WAL_DEBUG
  bool		XLOG_DEBUG = false;
*************** XLogFlush(XLogRecPtr record)
*** 2111,2116 ****
--- 2113,2132 ----
  			 */
  			continue;
  		}
+
+ 		/*
+ 		 * Sleep before flush! By adding a delay here, we may give further
+ 		 * backends the opportunity to join the backlog of group commit
+ 		 * followers; this can significantly improve transaction throughput, at
+ 		 * the risk of increasing transaction latency.
+ 		 *
+ 		 * We do not sleep if enableFsync is not turned on, nor if there are
+ 		 * fewer than CommitSiblings other backends with active transactions.
+ 		 */
+ 		if (CommitDelay > 0 && enableFsync &&
+ 			MinimumActiveBackends(CommitSiblings))
+ 			pg_usleep(CommitDelay);
+
  		/* Got the lock */
  		LogwrtResult = XLogCtl->LogwrtResult;
  		if (!XLByteLE(record, LogwrtResult.Flush))
diff src/backend/utils/misc/guc.c
index d75ab43..9b86ac1
*** a/src/backend/utils/misc/guc.c
--- b/src/backend/utils/misc/guc.c
*************** static struct config_int ConfigureNamesI
*** 2031,2037 ****
  	{
  		{"commit_delay", PGC_USERSET, WAL_SETTINGS,
  			gettext_noop("Sets the delay in microseconds between transaction commit and "
! 						 "flushing WAL to disk."),
  			NULL
  		},
  		&CommitDelay,
--- 2031,2037 ----
  	{
  		{"commit_delay", PGC_USERSET, WAL_SETTINGS,
  			gettext_noop("Sets the delay in microseconds between transaction commit and "
! 						 "flushing WAL to disk for the group commit leader."),
  			NULL
  		},
  		&CommitDelay,