Index: sys/ufs/ffs/ffs_vnops.c
===================================================================
--- sys/ufs/ffs/ffs_vnops.c
+++ sys/ufs/ffs/ffs_vnops.c
@@ -1001,7 +1001,9 @@
 			uio->uio_resid = resid;
 		}
 	} else if (resid > uio->uio_resid && (ioflag & IO_SYNC)) {
-		error = ffs_update(vp, 1);
+		if (!(ioflag & IO_DATASYNC) ||
+		    (ip->i_flags & (IN_SIZEMOD | IN_IBLKDATA)))
+			error = ffs_update(vp, 1);
 		if (ffs_fsfail_cleanup(VFSTOUFS(vp->v_mount), error))
 			error = ENXIO;
 	}