Index: swapon.c
===================================================================
--- swapon.c
+++ swapon.c
@@ -721,14 +721,39 @@
 	return (WEXITSTATUS(status));
 }
 
+static void
+swap_trim(const char *name)
+{
+
+	struct stat sb;
+	off_t ioarg[2];
+	int fd;
+
+	fd = open(name, O_RDONLY | O_DIRECT);
+	if (fd < 0)
+		return;
+	if (fstat(fd, &sb) >= 0) {
+		ioarg[0] = 0;	/* start address */
+		if (S_ISREG(sb.st_mode) || S_ISDIR(sb.st_mode)) {
+			ioarg[1] = sb.st_size;
+			ioctl(fd, DIOCGDELETE, ioarg);
+		} else if ((S_ISCHR(sb.st_mode) || S_ISBLK(sb.st_mode)) &&
+			   ioctl(fd, DIOCGMEDIASIZE, &ioarg[1]) >= 0)
+			ioctl(fd, DIOCGDELETE, ioarg);
+	}
+	close(fd);
+}
+
 static const char *
 swap_on_off_sfile(const char *name, int doingall)
 {
+
 	int error;
 
-	if (which_prog == SWAPON)
+	if (which_prog == SWAPON) {
+		swap_trim(name);
 		error = swapon(name);
-	else /* SWAPOFF */
+	} else /* SWAPOFF */
 		error = swapoff(name);
 
 	if (error == -1) {