Add C comment about why synchronous_commit=off behavior can lose

committed transactions in a postmaster crash.

src/backend/access/transam/xact.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.291 2010/05/13 11:39:30 sriggs Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.292 2010/06/29 18:44:58 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1028,7 +1028,7 @@ RecordTransactionCommit(void)
 	if (XactSyncCommit || forceSyncCommit || haveNonTemp)
 	{
 		/*
-		 * Synchronous commit case.
+		 * 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
@@ -1054,7 +1054,12 @@ RecordTransactionCommit(void)
 	else
 	{
 		/*
-		 * Asynchronous commit case.
+		 * Asynchronous commit case:
+		 *
+		 * This enables possible committed transaction loss in the case of a
+		 * postmaster crash because WAL buffers are left unwritten.
+		 * Ideally we could issue the WAL write without the fsync, but
+		 * some wal_sync_methods do not allow separate write/fsync.
 		 *
 		 * Report the latest async commit LSN, so that the WAL writer knows to
 		 * flush this commit.