unable to extend the volume

Issue

• We are using NFS backend for cinder. When I try to extend the volume size I get error Extend volume not implemented

[root@mac00163e747b4b ~(openstack_admin)]# cinder create 1 --volume-type nfs
+---------------------+--------------------------------------+
|       Property      |                Value                 |
+---------------------+--------------------------------------+
|     attachments     |                  []                  |
|  availability_zone  |                 nova                 |
|       bootable      |                false                 |
|      created_at     |      2015-07-29T13:23:44.125803      |
| display_description |                 None                 |
|     display_name    |                 None                 |
|      encrypted      |                False                 |
|          id         | 68105fcd-8758-4f7d-b668-27e27154e963 |
|       metadata      |                  {}                  |
|         size        |                  1                   |
|     snapshot_id     |                 None                 |
|     source_volid    |                 None                 |
|        status       |               creating               |
|     volume_type     |                 nfs                  |
+---------------------+--------------------------------------+


[root@mac00163e747b4b ~(openstack_admin)]# cinder list|grep -i 68105fcd-8758-4f7d-b668-27e27154e963
| 68105fcd-8758-4f7d-b668-27e27154e963 |    available    |       None       |  1   |     nfs     |  false   |             |

[root@mac00163e747b4b ~(openstack_admin)]# cinder extend 68105fcd-8758-4f7d-b668-27e27154e963  3

[root@mac00163e747b4b ~(openstack_admin)]# cinder list|grep -i 68105fcd-8758-4f7d-b668-27e27154e963
| 68105fcd-8758-4f7d-b668-27e27154e963 | error_extending |       None       |  1   |     nfs     |  false   |             |

[...]
2015-07-29 09:24:02.804 29636 INFO cinder.volume.manager [req-e791983d-0889-4b55-9ee8-48adf4a3facc f5bbaa4c14e64edcac86409c8b6531da 9498b1a74f9c442eb637f7d7c35d9587 - - -] volume 68105fcd-8758-4f7d-b668-27e27154e963: extending
2015-07-29 09:24:02.805 29636 ERROR cinder.volume.manager [req-e791983d-0889-4b55-9ee8-48adf4a3facc f5bbaa4c14e64edcac86409c8b6531da 9498b1a74f9c442eb637f7d7c35d9587 - - -] volume 68105fcd-8758-4f7d-b668-27e27154e963: Error trying to extend volume
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager Traceback (most recent call last):
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager   File "/usr/lib/python2.7/site-packages/cinder/volume/manager.py", line 1169, in extend_volume
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager     self.driver.extend_volume(volume, new_size)
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager   File "/usr/lib/python2.7/site-packages/cinder/volume/driver.py", line 531, in extend_volume
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager     raise NotImplementedError(msg)
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager NotImplementedError: Extend volume not implemented
2015-07-29 09:24:02.805 29636 TRACE cinder.volume.manager