B OpenNebula/one#6487

Updated SPARSE attribute description to make clear that QCOW2_STANDALONE is needed on system Datastore

Signed-off-by: Alejandro Mosteiro <amosteiro@opennebula.io>

source/open_cluster_deployment/storage_setup/nas_ds.rst

@@ -115,7 +115,7 @@ Additional Configuration
 * ``DD_BLOCK_SIZE``: Block size for `dd` operations (default: 64kB) could be set in ``/var/lib/one/remotes/etc/datastore/fs/fs.conf``.
 * ``SUPPORTED_FS``: Comma-separated list with every filesystem supported for creating formatted datablocks. Can be set in ``/var/lib/one/remotes/etc/datastore/datastore.conf``.
 * ``FS_OPTS_<FS>``: Options for creating the filesystem for formatted datablocks. Can be set in ``/var/lib/one/remotes/etc/datastore/datastore.conf`` for each filesystem type.
-* ``SPARSE``: If set to ``NO`` the images and disks in the image and system Datastore, respectively, wont be sparsed (i.e. the files will use all assigned space on the Datastore filesystem).
+* ``SPARSE``: If set to ``NO`` the images and disks in the image and system Datastore, respectively, wont be sparsed (i.e. the files will use all assigned space on the Datastore filesystem). It is mandatory to set ``QCOW2_STANDALONE = YES`` on the system Datastore for this setting to apply.
 * ``QCOW2_STANDALONE``: If set to ``YES`` the standalone qcow2 disk is created during :ref:`CLONE <clone>` operation (default: QCOW2_STANDALONE="NO"). Unlike previous options, this one is defined in image datastore template and inherited by the disks.
 
 .. warning:: Before adding a new filesystem to the ``SUPPORTED_FS`` list make sure that the corresponding ``mkfs.<fs_name>`` command is available in the Front-end and hypervisor Hosts. If an unsupported FS is used by the user the default one will be used.