Juju model migration fails during validation phase

[Erlon R. Cruz]

When trying to migrate a model between 2 controllers on Juju version
2.8.0, the migration fails due to error:

ERROR agents failed phase "validating" (machines: 1,2,3,5,8,9,10; units: calico/0,ceph-osd/1,containerd/0,containerd/1,etcd/0)
ERROR validating, some agents reported failure

This happens due to the rate-limiting on the target controller rejecting
the stampede of connections as all the agents start up again. This is
something model migration has not specifically been coded to handle.

To work around this issue, you need to set the following config on the
destination controller:

agent-ratelimit-max (default is 10): Should be set to a value at least equal
to the number of units (including subordinates) on the model being migrated.

[Salman (salmankh)]

How "agent-ratelimit-max" can be changed, can't find it in the controller or model config?

[Salman (salmankh)]

Ok, found that `juju controller-config` does not show the whole list of config parameters. However, it could be set via `juju controller-config -c <controller> agent-ratelimit-max=150`. Once set the controller-config starts showing agent-ratelimit-max value.

[Joseph Phillips (manadart)]

https://github.com/juju/juju/pull/12503 applies a retry strategy that better accommodates the default controller rate limiting.