Allow specifying disks as cache_devices instead of PVs

It might be hard for users to know names of the partitions we'll
create as PVs so we should let them disk names and get the PVs on
them ourselves when creating the cache.

README.md

@@ -226,8 +226,9 @@ e.g.: "30g", "50GiB".
 Mode for the cache. Supported values include `writethrough` (default) and `writeback`.
 
 #### `cache_devices`
-List of devices (physical volumes) that will be used for the cache. These should be
-physical volumes on fast SSD or NVMe drives.
+List of devices that will be used for the cache. These should be either physical volumes or
+drives these physical volumes are allocated on. Generally you want to select fast devices like
+SSD or NVMe drives for cache.
 
 #### `storage_safe_mode`
 When true (the default), an error will occur instead of automatically removing existing devices and/or formatting.

library/blivet.py

@@ -770,11 +770,17 @@ def _manage_cache(self):
             self._detach_cache()
 
     def _get_params_lvmcache(self):
+        parent = self._blivet_pool._device
         fast_pvs = []
-        for pv in self._volume['cache_devices']:
-            pv_device = self._blivet.devicetree.resolve_device(pv)
+        for cache_spec in self._volume['cache_devices']:
+            cache_device = self._blivet.devicetree.resolve_device(cache_spec)
+            if cache_device is None:
+                raise BlivetAnsibleError("cache device '%s' not found" % cache_spec)
+
+            pv_device = next((pv for pv in parent.pvs if cache_device.name in [an.name for an in pv.ancestors]),
+                             None)
             if pv_device is None:
-                raise BlivetAnsibleError("cache device '%s' not found" % pv)
+                raise BlivetAnsibleError("cache device '%s' doesn't seems to be a physical volume or its parent" % cache_spec)
             fast_pvs.append(pv_device)
 
         cache_request = devices.lvm.LVMCacheRequest(size=Size(self._volume['cache_size']),

tests/tests_create_lvm_cache_then_remove.yml

@@ -3,11 +3,12 @@
   become: true
   vars:
     storage_safe_mode: false
-    mount_location1: '/opt/test1'
-    mount_location2: '/opt/test2'
+    storage_use_partitions: true
     volume_group_size: '10g'
     volume_size: '5g'
     cache_size: '4g'
+  tags:
+    - tests::lvm
 
   tasks:
     - include_role:

tests/tests_fatals_cache_volume.yml

@@ -3,6 +3,11 @@
   become: true
   vars:
     storage_safe_mode: false
+    volume_group_size: '10g'
+    volume_size: '5g'
+    cache_size: '4g'
+  tags:
+    - tests::lvm
 
   tasks:
     - include_role:
@@ -18,6 +23,7 @@
     - include_tasks: get_unused_disk.yml
       vars:
         max_return: 2
+        disks_needed: 2
 
     - name: Verify that creating a cached partition volume fails
       block:
@@ -44,3 +50,30 @@
             that:
               - ansible_failed_result.msg != 'UNREACH'
             msg: "Role has not failed when it should have"
+
+    - name: Verify that creating cache on unused disk fails
+      block:
+        - name: Create cached volume
+          include_role:
+            name: linux-system-roles.storage
+          vars:
+            storage_pools:
+              - name: "foo"
+                disks: "{{ [unused_disks[0]] }}"
+                volumes:
+                  - name: test1
+                    size: "{{ volume_size }}"
+                    cached: true
+                    cache_size: "{{ cache_size }}"
+                    cache_devices: "{{ [unused_disks[1]] }}"
+
+        - name: unreachable task
+          fail:
+            msg: UNREACH
+
+      rescue:
+        - name: Check that we failed in the role
+          assert:
+            that:
+              - ansible_failed_result.msg != 'UNREACH'
+            msg: "Role has not failed when it should have"