If you get this error while trying to resize an openstack instance:

# nova resize fooinstance 16 --poll

==> /var/log/nova/nova-scheduler.log <==
2014-01-30 17:40:34 WARNING nova.scheduler.manager [req-aaaaaaa-bbbb-cccc-dddd-1ed34b64adef bajd7394hftgs71dba31d642342effa0f bfe2djhg6538sg384jgb82ks070ce0b] Failed to schedule_prep_resize: No valid host was found. 
2014-01-30 17:40:34 WARNING nova.scheduler.manager [req-aaaaaaa-bbbb-cccc-dddd-1ed34b64adef bajd7394hftgs71dba31d642342effa0f bfe2djhg6538sg384jgb82ks070ce0b] Setting instance aaaaaaaa-uuuu-dddd-bbbb-esdfg63asdfb to ACTIVE state.

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You have to set

--allow_resize_to_same_host=true

in /etc/nova/nova.conf and /etc/nova/nova-compute.conf and after that you need to restart the nova-compute and nova-api daemon.