First check the compute service in the compute node "service nova-compute status".

● nova-compute.service - OpenStack Compute Loaded: loaded (/lib/systemd/system/nova-compute.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2020-04-17 11:47:56 UTC; 20min ago Main PID: 5649 (nova-compute) Tasks: 28 (limit: 7372) CGroup: /system.slice/nova-compute.service ├─5649 /usr/bin/python3 /usr/bin/nova-compute --config-file=/etc/nova/nova.conf --config-file=/etc/nova/nova-compute.conf --log-file=/var/log/nova/nova-compute.log

Then check for the nova "/var/log/nova/nova-compute.log". Mostly you would find below :

2020-04-17 11:34:46.877 46192 ERROR nova.compute.manager [req-dde59938-b6ed-42e3-bf0c-4512176787e8 - - - - -] No compute node record for host svc-datarobot-srv-6: nova.exception_Remote.ComputeHostNotFound_Remote: Compute host svc-datarobot-srv-6 could not be found. 2020-04-17 11:34:46.881 46192 ERROR nova.compute.manager [req-dde59938-b6ed-42e3-bf0c-4512176787e8 - - - - -] Error updating resources for node svc-datarobot-srv-6.: FileNotFoundError: [Errno 2] No such file or directory: '/usr/lib/python3/dist-packages/instances'

The reason is a missing configuration in the nova.conf file. "state_path = /var/lib/nova" to be added in the [DEFAULT] section. & restart the service.

It seems that if the state_path value isn't provided, its default value will be the pybasedir, which is '/usr/lib/python2.7/dist-packages/instances in the error log.

Thanks to : https://ask.openstack.org/en/question/93321/nova-compute-state-is-down/?answer=104508#post-id-104508 (https://ask.openstack.org/en/question...)

So interesting! I have been spending hours (days soon) trying to resolve this. I deployed the rocky release using packstack on centos 7.5.1804 and an allinone deployment. packstack succeeds but I cannot spawn an instance. Tried to redeploy after setting selinux in permissive mode. same result.

[root@openstack nova(keystone_admin)]# /bin/sh -c "nova-manage cell_v2 discover_hosts --verbose" nova
Found 2 cell mappings.
Skipping cell0 since it does not contain hosts.
Getting computes from cell 'default': d26a5f4e-b59f-4e1b-808f-7caa29f9f5b2
Found 0 unmapped computes in cell: d26a5f4e-b59f-4e1b-808f-7caa29f9f5b2

Multiple ways of finding out the same thing...

[root@openstack nova(keystone_admin)]# nova-status upgrade check
+--------------------------------------------------------------------+
| Upgrade Check Results                                              |
+--------------------------------------------------------------------+
| Check: Cells v2                                                    |
| Result: Success                                                    |
| Details: No host mappings or compute nodes were found. Remember to |
|   run command 'nova-manage cell_v2 discover_hosts' when new        |
|   compute hosts are deployed.                                      |
+--------------------------------------------------------------------+
| Check: Placement API                                               |
| Result: Success                                                    |
| Details: None                                                      |
+--------------------------------------------------------------------+
| Check: Resource Providers                                          |
| Result: Success                                                    |
| Details: There are no compute resource providers in the Placement  |
|   service nor are there compute nodes in the database.             |
|   Remember to configure new compute nodes to report into the       |
|   Placement service. See                                           |
|   https://docs.openstack.org/nova/latest/user/placement.html       |
|   for more details.                                                |
+--------------------------------------------------------------------+

From nova-placement-api.log:

2018-11-22 16:25:02.813 28109 INFO nova.scheduler.host_manager [req-624b4df5-30e7-4eb5-bbff-f7d14f50fb13 - - - - -] Host mapping not found for host openstack.frippe.com. Not tracking instance info for this host.
2018-11-22 16:25:02.813 28109 INFO nova.scheduler.host_manager [req-624b4df5-30e7-4eb5-bbff-f7d14f50fb13 - - - - -] Received a sync request from an unknown host 'openstack.xxx.com'. Re-created its InstanceList.
2018-11-22 17:52:12.870 28109 INFO nova.scheduler.manager [req-d7b6724d-7406-4cf6-a747-f428c49aa302 e763008d9b3e49e2803d39ad1d43696e d365b3a65ba6428f86119b6f84fac7e8 - default default] Got no allocation candidates from the Placement API. This could be due to insufficient resources or a temporary occurrence as compute nodes start up.
2018-11-23 04:59:54.091 28108 INFO nova.scheduler.manager [req-1c63094b-0c63-4b26-ad1f-865777293c8e e763008d9b3e49e2803d39ad1d43696e d365b3a65ba6428f86119b6f84fac7e8 - default default] Got no allocation candidates from the Placement API. This could be due to insufficient resources or a temporary occurrence as compute nodes start up.

I do get something from service list:

[root@openstack nova(keystone_admin)]# openstack compute service list --service nova-compute
+----+--------------+----------------------+------+---------+-------+----------------------------+
| ID | Binary       | Host                 | Zone | Status  | State | Updated At                 |
+----+--------------+----------------------+------+---------+-------+----------------------------+
| 10 | nova-compute | openstack.xxx.com | nova | enabled | up    | 2018-11-23T04:14:33.000000 |
+----+--------------+----------------------+------+---------+-------+----------------------------+

https://docs.openstack.org/nova/train/install/compute-install-rdo.html#add-the-compute-node-to-the-cell-database (https://docs.openstack.org/nova/train...)

try in controller su -s /bin/sh -c "nova-manage cell_v2 discover_hosts --verbose" nova

and then nova-status upgrade check

ADD below lines in /etc/nova/nova.conf file in controller.

[scheduler]
discover_hosts_in_cells_interval = 300

then try to run all database sync command and run the su -s /bin/sh -c "nova-manage cell_v2 discover_hosts --verbose" nova

then restart all compute services.

then run

openstack compute service list

this will surely solve your problem.

I am also facing same issue, whether this one is solved ??

still it is not working.

@frippe75 Try to add the compute node in database manually,like below mentioned

In nova api-database, there is a table called host_mappings will there, you can manually add your compute node in that table