Cinder multi-backend with multiple Ceph pools

This isn't so much of a 'how-to' (as it's been documented with perfect clarity by Sébastien Han here), it's more of a warning when enabling the multi-backend functionality. tl;dr If you enable Cinder multi-backend, double-check the output of cinder service-list and be sure to update the host mappings for all your existing volumes.

I came across a problem recently after enabling Cinder's multi-backend feature to support multiple Ceph pools. Attempting to delete existing instances that had volumes associated with them would fail; The symptom was a HTTP/504 (gateway timeout) leaving the machine in an 'error' state having sat there 'deleting' for some time. Instantiating new virtual machines, attaching volumes, and then deleting them was all fine.

For the failed deletions, as far as Nova's logs are concerned the delete mostly goes according to plan apart from the call to Cinder to detach the associated volume:

DEBUG cinderclient.client [req-3011c2ec-2274-49e4-9f1e-e9ec7eb850a5 ] Failed attempt(1 of
3), retrying in 1 seconds _cs_request

And a few seconds later after the third attempt, the following exception makes an appearance:

ERROR oslo.messaging.rpc.dispatcher [req-3011c2ec-2274-49e4-9f1e-e9ec7eb850a5 ] Exception
during message handling: Gateway Time-out (HTTP 504)

This is where our instance is left in the error state. Tracing through from the original request using the request-id (yay for ELK) Cinder seemingly gives up and the RPC request times out. We can see where Nova talks to Cinder API, for example:

INFO cinder.api.openstack.wsgi [req-c0854007-3b72-4964-b337-b9e331874c4e
d1a60c7b4bbc427e8f2c3f99d0c2e6d2 509d6ae853114ea9aaaac02804d3a4dd - - -] POST

There's the message generated:

DEBUG oslo_messaging._drivers.amqpdriver [req-c0854007-3b72-4964-b337-b9e331874c4e
d1a60c7b4bbc427e8f2c3f99d0c2e6d2 509d6ae853114ea9aaaac02804d3a4dd - - -] MSG_ID is
774b48ebff5540fcb9262f35941f6f9d _send

DEBUG oslo_messaging._drivers.amqp [req-c0854007-3b72-4964-b337-b9e331874c4e
d1a60c7b4bbc427e8f2c3f99d0c2e6d2 509d6ae853114ea9aaaac02804d3a4dd - - -] UNIQUE_ID is
a39a4a8ec681468b87463968a8b64a2c. _add_unique_id

And then a few seconds later we see another exception:

ERROR cinder.api.middleware.fault [req-c0854007-3b72-4964-b337-b9e331874c4e
d1a60c7b4bbc427e8f2c3f99d0c2e6d2 509d6ae853114ea9aaaac02804d3a4dd - - -] Caught error: Timed
out waiting for a reply to message ID 774b48ebff5540fcb9262f35941f6f9d


INFO eventlet.wsgi.server [req-c0854007-3b72-4964-b337-b9e331874c4e d1a60c7b4bbc427e8f2c3f99d0c2e6d2 509d6ae853114ea9aaaac02804d3a4dd - - -] Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/eventlet/", line 468, in handle_one_response
  File "/usr/lib/python2.7/dist-packages/eventlet/", line 399, in write
  File "/usr/lib/python2.7/", line 334, in writelines
  File "/usr/lib/python2.7/", line 303, in flush
  File "/usr/lib/python2.7/dist-packages/eventlet/", line 376, in sendall
    tail = self.send(data, flags)
  File "/usr/lib/python2.7/dist-packages/eventlet/", line 358, in send
    total_sent += fd.send(data[total_sent:], flags)
error: [Errno 104] Connection reset by peer

There were no other problems on the platform at the time - loadbalancers are fine, RabbitMQ is OK - and so on the face of it this is something of a mystery.

My initial attempts at triaging the issue involved manually trying to detach the volume associated with the instance:

❯ nova reset-state --active 75966ed2-e003-4575-8b64-08c4c414492c
Reset state for server 75966ed2-e003-4575-8b64-08c4c414492c succeeded; new state is active

❯ nova stop 75966ed2-e003-4575-8b64-08c4c414492c
Request to stop server 75966ed2-e003-4575-8b64-08c4c414492c has been accepted.

❯ nova volume-detach 75966ed2-e003-4575-8b64-08c4c414492c

❯ cinder show 7a8a46ba-868a-4620-acc5-84467036fecb | grep -i status
| status | detaching |

And there it hung, indefinitely. Googling the symptoms of the problem don't necessarily get you very far at this point, as there's no shortage of bugs out there to do with Cinder volumes ending up in a strange state and instances that can't be deleted. Most of these suggest diving into the database in order to clean things up but really you should try and discern the root cause if it becomes apparent that this isn't a one off.

The clue really is in the fact that there's some kind of messaging timeout. We know that cinder-api is receiving the request from nova-compute, so what's happening to the message? Why isn't cinder-volume picking it up? Well, every service on OpenStack has a registry of what runs where in terms of agents, whether it's nova-conductor, nova-compute, neutron-dhcp-agent, neutron-metadata-agent, etc., and the same is true of Cinder - there's cinder-volume and cinder-scheduler. Knowing that there'd been some very recent changes to Cinder's configuration, on a hunch I decided to check the output of cinder service-list:

❯ cinder service-list
|      Binary      |                      Host                     |  Status  | State |
| cinder-scheduler |                   controller0                 | enabled  |   up  |
| cinder-scheduler |                   controller1                 | enabled  |   up  |
|  cinder-volume   |                   controller0                 | enabled  |  down |
|  cinder-volume   |                   controller1                 | enabled  |  down |
|  cinder-volume   | rbd:cinder.volumes.flash@cinder.volumes.flash | enabled  |   up  |
|  cinder-volume   |       rbd:cinder.volumes@cinder.volumes       | enabled  |   up  |

And there is our smoking gun. Enabling the multi-backend functionality has introduced two new instances of cinder-volume, one per pool, and now the previous instances with the old configuration that were responsible for talking to Ceph are now 'down' - that's why the RPC messages from cinder-api aren't being responded to, because the 'host' responsible for that volume is AWOL.

In order to fix this there's a couple of things that need to be done. First is to administratively disable the older, now redundant, services:

❯ for host in controller{0,1}; do cinder service-disable $host cinder-volume ; done
|    Host     |     Binary    |  Status  |
| controller0 | cinder-volume | disabled |
|    Host     |     Binary    |  Status  |
| controller1 | cinder-volume | disabled |

The next step is to update the mappings for the volumes themselves, and unfortunately to do that we need to make some changes in the Cinder database. The first change is to the volumes table, and the second is to the volume_mappings table - both of these will update the associated host column for the row that corresponds to the volume_id associated with this instance.

MariaDB [cinder]> select host from volumes where id='7a8a46ba-868a-4620-acc5-84467036fecb';
| host                |
| controller1#DEFAULT |

MariaDB [cinder]> select volume_id,attached_host,instance_uuid from volume_attachment where
| volume_id                            | attached_host       | instance_uuid                        |
| 7a8a46ba-868a-4620-acc5-84467036fecb | controller1#DEFAULT | 75966ed2-e003-4575-8b64-08c4c414492c |

The convention when updating the host and attached_host columns is to use host#pool, so to fix these rows we need to do the following:

MariaDB [cinder]> update volumes set host='rbd:cinder.volumes@cinder.volumes#cinder.volumes'
where id='7a8a46ba-868a-4620-acc5-84467036fecb' limit 1;
MariaDB [cinder]> update volume_attachment set
attached_host='rbd:cinder.volumes@cinder.volumes#cinder.volumes' where
volume_id='7a8a46ba-868a-4620-acc5-84467036fecb' limit 1;

Now let's try deleting our instances again:

❯ cinder reset-state --state in-use 7a8a46ba-868a-4620-acc5-84467036fecb
❯ nova delete 75966ed2-e003-4575-8b64-08c4c414492c
Request to delete server 75966ed2-e003-4575-8b64-08c4c414492c has been accepted.

And a couple of seconds later:

❯ nova show 75966ed2-e003-4575-8b64-08c4c414492c
ERROR (CommandError): No server with a name or ID of '75966ed2-e003-4575-8b64-08c4c414492c'

Hurrah! If that works you'll need to update both the volumes and volume_attachments table en masse to reflect the new hosts, and then delete the service entries for the redundant services themselves.