@olivierlambert Good question,
Once I booted it back, it added as a slave to the pool with out issues.

But I have removed it from the pool and am re-installing as we speak.
We're moving data centres, so I'm splitting the pool up to do a part-move.

Thanks for your interest, and assistance.

Okay Team,
For my reference, and anyone else finding this;

I was able to recover the pool by completing the following steps.
As this is out production cluster, I did it late into the night - thankfully, with no downtime.

1: I shutdown the old master than had no VM's running.

1: On another host that I wanted to become the master, I ssh'd on and ran the following command.
xe pool-emergency-transition-to-master

I got the following response:

Host agent will restart and transition to master in 10.000 seconds...

3: I then went into XO => Settings => Server and connected to the new master.

2: After a god 10-15 seconds, On the new master I ran the following command
xe pool-recover-slaves

Which after a few seconds, the GUID's of the other hosts in the pool appeared.

xe pool-recover-slaves
f600ea3a-cc02-4f24-a15e-938756feb00c
d123525c-6022-43c9-8c23-d0f1ca567219
16c7c955-f65e-4d5b-96e7-787085c2d25f

And all hosts are showing correctly in XO!

@olivierlambert Yes, I updated the master and rebooted it first.
Then updated the other hosts and rebooted each of those.

I've found this post:
https://docs.xenserver.com/en-us/citrix-hypervisor/dr/machine-failures.html#master-failures

Which looks like I can run

xe pool-emergency-transition-to-master

On one of the other members of the pool.
Is this the best/safest way to change the pool master?

@olivierlambert Do you think this is a solution to the errors?

Thanks

Hi Team,
I want to remove the master from our production pool.

I get the following error:

xe pool-designate-new-master host-uuid=<<other pool server uuid>>

The server failed to handle your request, due to an internal error. The given message may give details useful for debugging the problem.
message: missing column
<extra>: host
<extra>: https_only

I have made sure that the current Pool master was fully updated first, and rebooted.
I then updated all other hosts in the pool (5 in total).

yum update
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
Excluding mirror: updates.xcp-ng.org
 * xcp-ng-base: mirrors.xcp-ng.org
Excluding mirror: updates.xcp-ng.org
 * xcp-ng-updates: mirrors.xcp-ng.org
No packages marked for update

I have even shutdown the master, loosing connection via XO and running the change master command via the command line on the other servers, which of course, do nothing.

I have no VM's on the current pool master, and have tried put in maintainence mode first.

In XO (from courses) I have tried to Detach the master which generates the following error in the logs;

host.detach
{
  "host": "d2d01b9a-e6d1-4dd0-90e6-f101a251c7b1"
}
{
  "code": "INTERNAL_ERROR",
  "params": [
    "Xapi_pool.Cannot_eject_master"
  ],
  "call": {
    "method": "pool.eject",
    "params": [
      "OpaqueRef:90ba839f-61a2-4f96-b485-df6332fb0bce"
    ]
  },
  "message": "INTERNAL_ERROR(Xapi_pool.Cannot_eject_master)",
  "name": "XapiError",
  "stack": "XapiError: INTERNAL_ERROR(Xapi_pool.Cannot_eject_master)
    at Function.wrap (/opt/xo/xo-builds/xen-orchestra-202310302357/packages/xen-api/src/_XapiError.js:16:12)
    at /opt/xo/xo-builds/xen-orchestra-202310302357/packages/xen-api/src/transports/json-rpc.js:35:21
    at runNextTicks (node:internal/process/task_queues:60:5)
    at processImmediate (node:internal/timers:447:9)
    at process.callbackTrampoline (node:internal/async_hooks:130:17)"
}

I have run xe host-list to get the other host UUID's from all 5 servers and have tried the xe pool-designate-new-master host-uuid= command on the current master and all other servers with the same error message.

I have even tried the window controller MSI via the gui

Which appears to timeout and fails.

Host list:

XO Errors Screen: