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Disable the node so that no new VMs will be scheduled on this host. Instructions: Disable/Enable a compute node howto
Migrate the instances on this compute node to other hypervisors. Instructions: Migrate an instance howto (warning: there was once a problem with live-block migration; we therefore decided to always use cold migration)
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Reinstall the host with CentOS8 stream using Foreman and move the host to the 'hosts_all' hostgroup
As Operating System choose: CentOS Linux 8.2.2004Stream 8
As Media choose: CentOS-Stream-local
As Partition table select
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