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Just to share our config, we don't use "backend_host" but only "host"  
on all control nodes. Its value is the hostname (shortname) pointing  
to the virtual IP which migrates in case of a failure. We only use  
Ceph as storage backend with different volume types. The volume  
service list looks like this:

controller02:~ # openstack volume service list
+------------------+--------------------+------+---------+-------+----------------------------+
| Binary           | Host               | Zone | Status  | State |  
Updated At                 |
+------------------+--------------------+------+---------+-------+----------------------------+
| cinder-scheduler | controller         | nova | enabled | up    |  
2023-08-30T10:32:36.000000 |
| cinder-backup    | controller         | nova | enabled | up    |  
2023-08-30T10:32:34.000000 |
| cinder-volume    | controller at rbd     | nova | enabled | up    |  
2023-08-30T10:32:28.000000 |
| cinder-volume    | controller at rbd2    | nova | enabled | up    |  
2023-08-30T10:32:36.000000 |
| cinder-volume    | controller at ceph-ec | nova | enabled | up    |  
2023-08-30T10:32:28.000000 |
+------------------+--------------------+------+---------+-------+----------------------------+

We haven't seen any issue with this setup in years during failover.

Zitat von Gorka Eguileor <geguileo at redhat.com>:

> On 29/08, Albert Braden wrote:
>>  What does backend_host look like? Should it match my internal API  
>> URL, i.e. api-int.qde4.ourdomain.com?
>>      On Tuesday, August 29, 2023 at 10:44:48 AM EDT, Gorka Eguileor  
>> <geguileo at redhat.com> wrote:
>>
>
> Hi,
>
> If I remember correctly you can set it to anything you want, but for
> convenience I would recommend setting it to the hostname that currently
> has more volumes in your system.
>
> Let's say you have 3 hosts:
>   qde4-ctrl1.cloud.ourdomain.com
>   qde4-ctrl2.cloud.ourdomain.com
>   qde4-ctrl3.cloud.ourdomain.com
>
> And you have 100 volumes on ctrl1, 20 on ctrl2, and 10 on ctrl3.
>
> Then it would be best to set it to ctrl1:
>  [rbd-1]
>  backend_host = qde4-ctrl1.cloud.ourdomain.com
>
> And then use the cinder-manage command to modify the other 2.
>
> For your information the value you see as "os-vol-host-attr:host" when
> seeing the detailed information of a volume is in the form of:
> <HostName>@<BackendName>#<PoolName>
>
> In your case:
>   <HostName> = qde4-ctrl1.cloud.ourdomain.com
>   <BackendName> = rbd-1
>   <PoolName> = rbd-1                         
>
> In the RBD case the poolname will always be the same as the backendname.
>
> Cheers,
> Gorka.
>
>>  On 29/08, Albert Braden wrote:
>> > We’re replacing controllers, and it takes a few hours to build  
>> the new controller. We’re following this procedure to remove the  
>> old controller:  
>> https://docs.openstack.org/kolla-ansible/train/user/adding-and-removing-hosts.html
>> >
>> >  After that the cluster seems to run fine on 2 controllers, but  
>> approximately 1/3 of our volumes can’t be attached to a VM. When we  
>> look at those volumes, we see this:
>> >
>> > | os-vol-host-attr:host          |  
>> qde4-ctrl1.cloud.ourdomain.com at rbd-1#rbd-1                           
>>   |
>> >
>> > Ctrl1 is the controller that is being replaced. Is it possible to  
>> change the os-vol-host-attr on a volume? How can we work around  
>> this issue while we are replacing controllers? Do we need to  
>> disable the API for the duration of the replacement process, or is  
>> there a better way?
>> >
>>
>> Hi,
>>
>> Assuming you are running cinder volume in Active-Passive mode (which I
>> believe was the only way back in Train) then you should be hardcoding
>> the host name in the cinder.conf file to avoid losing access to your
>> volumes when the volume service starts in another host.
>>
>> This is done with the "backend_host" configuration option within the
>> specific driver section in cinder.conf.
>>
>> As for how to change the value of all the volumes to the same host
>> value, you can use the "cinder-manage" command:
>>
>>   cinder-manage volume update_host \
>>     --currenthost <current host> \
>>     --newhost <new host>
>>
>> Cheers,
>> Gorka.
>>
>>
>>