Deprecate sparse LVs

That feature was never tested in Nova and we had some bugs. Deprecating it.

Change-Id: I6f05bb32313de1d832fe080c00a74508224acd77

nova/conf/libvirt.py

@@ -665,6 +665,13 @@ Related options:
 """),
     cfg.BoolOpt('sparse_logical_volumes',
                 default=False,
+                deprecated_for_removal=True,
+                deprecated_since='18.0.0',
+                deprecated_reason="""
+Sparse logical volumes is a feature that is not tested hence not supported.
+LVM logical volumes are preallocated by default. If you want thin provisioning,
+use Cinder thin-provisioned volumes.
+""",
                 help="""
 Create sparse logical volumes (with virtualsize) if this flag is set to True.
 """),

nova/virt/libvirt/imagebackend.py

@@ -685,8 +685,8 @@ class Lvm(Image):
 
         super(Lvm, self).__init__(path, "block", "raw", is_block_dev=True)
 
-        # TODO(pbrady): possibly deprecate libvirt.sparse_logical_volumes
-        # for the more general preallocate_images
+        # TODO(sbauza): Remove the config option usage and default the
+        # LVM logical volume creation to preallocate the full size only.
         self.sparse = CONF.libvirt.sparse_logical_volumes
         self.preallocate = not self.sparse
 

nova/virt/libvirt/storage/lvm.py

@@ -35,6 +35,7 @@ CONF = nova.conf.CONF
 LOG = logging.getLogger(__name__)
 
 
+# TODO(sbauza): Remove the possibility to ask for a sparse LV.
 def create_volume(vg, lv, size, sparse=False):
     """Create LVM image.
 

releasenotes/notes/deprecate_sparse_lvs-99f30d70a68a028d.yaml

@@ -0,0 +1,10 @@
+---
+deprecations:
+  - |
+    The ``[libvirt]/sparse_logical_volumes`` configuration option is now
+    deprecated. Sparse logical volumes were never verified by tests in Nova
+    and some bugs were found without having fixes so we prefer to deprecate
+    that feature.
+    By default, the LVM image backend allocates all the disk size to a logical
+    volume. If you want to have the volume group having thin-provisioned
+    logical volumes, use Cinder with volume-backed instances.