Stole from https://forum.proxmox.com/threads/ceph-osd-on-lvm-logical-volume.68618/
1. During install set maxvz to 0 to not create local storage and keep free space for Ceph on the OS drive. [GUIDE, 2.3.1 Advanced LVM Configuration Options ]
2. Setup Proxmox like usual and create a cluster
3. Install