>my ovirt cluster has got 3 Hypervisors runnig Centos 7.5.1804 vdsm is 4.20.39.1-1.el7,
>ovirt engine is 4.2.4.5-1.el7, the storage systems are HP MSA P2000 and 2050 (fibre channel).

>I need to stop one of the hypervisors for maintenance but this system is the storage pool manager.

>For this reason I decided to manually activate SPM in one of the other nodes but this operation is not
successful.

Why would you do that ?

Just put the host in maintenance and the system will evacuate all VMs and another host will be promoted as SPM . 

Once the host is in maintenance - you can proceed with your tasks.

P.S: Don't forget to remove the mainteance mode once you are over.

Best Regards,

Strahil Nikolov