Sorry Marian, you are quite right. Ok, I was testing on mitaka and this is evidently a bug on pike+. In pike cinder uses thin provisioning by default with the lvm backend so a lv pool is automatically created that does not correspond to a specific cinder volume. Before any cinder volumes have been provisioned:


ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "pvs"
  PV         VG             Fmt  Attr PSize PFree  
  /dev/vdd   cinder-volumes lvm2 a--  3.00g 140.00m

ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "vgs"
  VG             #PV #LV #SN Attr   VSize VFree  
  cinder-volumes   1   1   0 wz--n- 3.00g 140.00m

ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "lvs"
  LV                  VG             Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  cinder-volumes-pool cinder-volumes twi-a-tz-- 2.85g             0.00   1.07  
                          

Creating a cinder volume creates a lv inside the logical thin pool:

ubuntu@gnuoy-bastion:~/bugs/1735931⟫ cinder create 1
+--------------------------------+--------------------------------------+
| Property                       | Value                                |
+--------------------------------+--------------------------------------+
| attachments                    | []                                   |
| availability_zone              | nova                                 |
| bootable                       | false                                |
| consistencygroup_id            | None                                 |
| created_at                     | 2017-12-05T17:35:02.000000           |
| description                    | None                                 |
| encrypted                      | False                                |
| id                             | 0f2c6b0a-09c7-4bb8-b5a8-718d7a4cfd4f |
| metadata                       | {}                                   |
| migration_status               | None                                 |
| multiattach                    | False                                |
| name                           | None                                 |
| os-vol-host-attr:host          | None                                 |
| os-vol-mig-status-attr:migstat | None                                 |
| os-vol-mig-status-attr:name_id | None                                 |
| os-vol-tenant-attr:tenant_id   | 5087487c5630440e955f5b33d445fe9f     |
| replication_status             | None                                 |
| size                           | 1                                    |
| snapshot_id                    | None                                 |
| source_volid                   | None                                 |
| status                         | creating                             |
| updated_at                     | None                                 |
| user_id                        | 89ea60545efd4752b4cc90329958b698     |
| volume_type                    | None                                 |
+--------------------------------+--------------------------------------+

ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "lvs"
  LV                                          VG             Attr       LSize Pool                Origin Data%  Meta%  Move Log Cpy%Sync Convert
  cinder-volumes-pool                         cinder-volumes twi-aotz-- 2.85g                            0.00   1.17                            
  volume-0f2c6b0a-09c7-4bb8-b5a8-718d7a4cfd4f cinder-volumes Vwi-a-tz-- 1.00g cinder-volumes-pool        0.00  


And as Marian said, if an additional device is added the thin pool is not extended:

ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju config cinder block-device="/dev/vdd /dev/vde"
ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "pvs"
  PV         VG             Fmt  Attr PSize PFree  
  /dev/vdd   cinder-volumes lvm2 a--  3.00g 140.00m
  /dev/vde   cinder-volumes lvm2 a--  3.00g   3.00g
ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "vgs"
  VG             #PV #LV #SN Attr   VSize VFree
  cinder-volumes   2   2   0 wz--n- 5.99g 3.13g
ubuntu@gnuoy-bastion:~/bugs/1735931⟫ juju run --unit cinder/0 "lvs"
  LV                                          VG             Attr       LSize Pool                Origin Data%  Meta%  Move Log Cpy%Sync Convert
  cinder-volumes-pool                         cinder-volumes twi-aotz-- 2.85g                            0.00   1.17                            
  volume-0f2c6b0a-09c7-4bb8-b5a8-718d7a4cfd4f cinder-volumes Vwi-a-tz-- 1.00g cinder-volumes-pool        0.00