diff --git a/fish/options.c b/fish/options.c
index cfdbdfec7..80b71ecfd 100644
--- a/fish/options.c
+++ b/fish/options.c
@@ -134,6 +134,10 @@ add_drives_handle (guestfs_h *g, struct drv *drv, char next_drive)
         ad_optargs.bitmask |= GUESTFS_ADD_DRIVE_OPTS_CACHEMODE_BITMASK;
         ad_optargs.cachemode = drv->a.cachemode;
       }
+      if (drv->a.discard) {
+        ad_optargs.bitmask |= GUESTFS_ADD_DRIVE_OPTS_DISCARD_BITMASK;
+        ad_optargs.discard = drv->a.discard;
+      }
 
       r = guestfs_add_drive_opts_argv (g, drv->a.filename, &ad_optargs);
       if (r == -1)
@@ -279,6 +283,7 @@ free_drives (struct drv *drv)
     free (drv->a.filename);
     /* a.format is an optarg, so don't free it */
     /* a.cachemode is a static string, so don't free it */
+    /* a.discard is a static string, so don't free it */
     break;
   case drv_uri:
     free (drv->uri.path);
diff --git a/fish/options.h b/fish/options.h
index a7b8277f9..dbf9163ea 100644
--- a/fish/options.h
+++ b/fish/options.h
@@ -61,6 +61,7 @@ struct drv {
       char *filename;       /* disk filename */
       const char *format;   /* format (NULL == autodetect) */
       const char *cachemode;/* cachemode (NULL == default) */
+      const char *discard;  /* discard (NULL == disable) */
     } a;
     struct {
       char *path;           /* disk path */
diff --git a/format/format.c b/format/format.c
index cff0d8252..684bffd1a 100644
--- a/format/format.c
+++ b/format/format.c
@@ -188,6 +188,11 @@ main (int argc, char *argv[])
 
     case 'a':
       OPTION_a;
+
+      /* Enable discard on all drives added on the command line. */
+      assert (drvs != NULL);
+      assert (drvs->type == drv_a);
+      drvs->a.discard = "besteffort";
       break;
 
     case 'v':
@@ -342,6 +347,14 @@ do_format (void)
     }
   }
 
+  /* Send TRIM/UNMAP to all block devices, to give back the space to
+   * the host.  However don't fail if this doesn't work.
+   */
+  guestfs_push_error_handler (g, NULL, NULL);
+  for (i = 0; devices[i] != NULL; ++i)
+    guestfs_blkdiscard (g, devices[i]);
+  guestfs_pop_error_handler (g);
+
   if (do_rescan (devices))
     return 1; /* which means, reopen the handle and retry */