Very nicely put MO! Ceph indeed diminishes the need to have RAID setup. I myself use combination of RAID and Expander card to have JBOD for Ceph. I am not sure if battery backed cache is needed since Ceph can heal itself pretty good. This is was my primary concern when i was introduced to Ceph. I ran several tests to simulate complete Ceph node failure but never had any data issue. If we are talking about Caching through Proxmox for VMs such as writeback, write through etc. of course they are little different story. They really have nothing to do with RAID cache we are talking about here.