Open Stack

I am have build 15 node openstack cluster and now I have 5 node for ceph storage they all are HP DL360p G8  which has 10 HDD tray. 

Now for ceph minimum requirement is to have 3 monitor node, I have 5 ceph node then it would be little tight if I give 3 node for monitor and 2 for OSD. 

I was thinking can we use 3 infra* node for monitor from openstack-ansible? 

How much risk is there to use ceph mon node on infra node and how much memory cpu monitor node need ? 

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