When it comes to designing a vSphere environment, a lot of topics come into play in deciding how to provision storage to a virtual machine (VM). One decision point is whether or not to use Raw Device Mappings or RDMs. RDMs allow a special file in a VMFS volume to act as a proxy for a storage resource. The logical unit number, or LUN, that is a candidate for an RDM is attached to an individual VM; rather than formatted for the entire cluster. The benefits of using an RDM are for clustering support requirements and to allow disk commands to be sent directly to the storage resource from a VM.
Simply put, RDMs complicate the backups of VMs. In the case of Veeam Backup & Replication, a few guidelines need to be kept in mind to ensure that a VM with one or more RDMs can be backed up. VMs with physical RDMs and independent disks cannot be backed up with Veeam Backup & Replication. In both cases, this is because VM snapshots cannot be applied. Veeam Backup & Replication performs its backup and replication tasks by issuing a VMware snapshot to the VM. Defining how an RDM is set up on a VM is decided during the add hardware wizard.