Ceph is a kind of a distributed object and file storage system, which fully integrates with Proxmox. Out of the box, Proxmox comes with the Ceph cluster management option through the GUI and a whole array of features to make the integration as seamless as possible. We will dive deep into Ceph in Chapter 4, Storage Systems. Ceph can be installed on its own hardware using operating systems such as Ubuntu, or it can coexist with Proxmox on the same node. Whether it's coexisting or on its own cluster, Proxmox nodes need access to the Ceph configuration file to connect. The configuration file is located in /etc/pve/ceph.conf for the Proxmox+Ceph coexisting node. For non-coexisting Proxmox nodes, the file needs to be stored in /etc/ceph/ceph.conf. In the coexisting node, Proxmox creates a symbolic link of the Ceph configuration file in /etc/ceph/ceph.conf.

Besides the configuration file, Ceph also uses authentication keys, which are stored in the following directories:

/etc/pve/priv/ceph.client.admin.keyring 
/etc/pve/priv/ceph.mon.keyring 
/etc/pve/priv/ceph/<rbd_storage_id>.keyring 

In order to connect a Ceph RBD storage, Proxmox requires a separate keyring. The <rbd_storage_id>.keyring is simply a copied and renamed version of ceph.client.admin.keyring. Without this keyring, Proxmox will not be able to connect to Ceph. We will look at details of Ceph in Chapter 5, Installing and Configuring Ceph.