nspawn: if we set up a loopback device, try to mount it with "discard"

Let's make sure that our loopback files remain sparse, hence let's set
"discard" as mount option on file systems that support it if the backing device
is a loopback.

diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index 2952938..c56af6e 100644 (file)
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -2260,7 +2260,7 @@ static int dissect_image(
 static int mount_device(const char *what, const char *where, const char *directory, bool rw) {
 #ifdef HAVE_BLKID
         _cleanup_blkid_free_probe_ blkid_probe b = NULL;
-        const char *fstype, *p;
+        const char *fstype, *p, *options;
         int r;
 
         assert(what);
@@ -2309,7 +2309,17 @@ static int mount_device(const char *what, const char *where, const char *directo
                 return -EOPNOTSUPP;
         }
 
-        return mount_verbose(LOG_ERR, what, p, fstype, MS_NODEV|(rw ? 0 : MS_RDONLY), NULL);
+        /* If this is a loopback device then let's mount the image with discard, so that the underlying file remains
+         * sparse when possible. */
+        if (STR_IN_SET(fstype, "btrfs", "ext4", "vfat", "xfs")) {
+                const char *l;
+
+                l = path_startswith(what, "/dev");
+                if (l && startswith(l, "loop"))
+                        options = "discard";
+        }
+
+        return mount_verbose(LOG_ERR, what, p, fstype, MS_NODEV|(rw ? 0 : MS_RDONLY), options);
 #else
         log_error("--image= is not supported, compiled without blkid support.");
         return -EOPNOTSUPP;