diff --git a/contrib/pg_test_fsync/pg_test_fsync.c b/contrib/pg_test_fsync/pg_test_fsync.c
new file mode 100644
index 669c419..682821a
*** a/contrib/pg_test_fsync/pg_test_fsync.c
--- b/contrib/pg_test_fsync/pg_test_fsync.c
*************** test_sync(int writes_per_op)
*** 369,374 ****
--- 369,380 ----
  		{
  			for (writes = 0; writes < writes_per_op; writes++)
  				if (write(tmpfile, buf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+ 				/*
+ 				 * This can generate write failures if the filesystem
+ 				 * has a large block size, e.g. 4k, and there is no
+ 				 * support for O_DIRECT writes smaller than the
+ 				 * file system block size, e.g. XFS.
+ 				 */
  					die("write failed");
  			if (lseek(tmpfile, 0, SEEK_SET) == -1)
  				die("seek failed");