3.5 RDM, PMem, NVMe, NVMe-oF & iSER Use Cases

3.5 RDM, PMem, NVMe, NVMe-oF & iSER Use Cases

Most VMs store their disks as VMDK files on VMFS, NFS, vSAN, or vVols. Objective 1.3.6 covers the special-purpose options you pick when a workload needs direct LUN access, persistent memory, or the lowest-latency transports.

Raw Device Mapping (RDM)

An RDM is a mapping file (a small .vmdk pointer) on a VMFS datastore that gives a VM direct access to a whole LUN. The VM's I/O goes to the LUN, while the mapping file lets vCenter manage it like a disk.

Use cases for RDMs:

• Windows Server Failover Clustering (WSFC) across hosts with shared disks that use SCSI-3 persistent reservations (pRDM).
• SAN management or array-aware software in the guest that must send commands directly to the LUN, such as array snapshot agents.
• Physical-to-virtual flexibility, where a LUN may need to be presented to a physical server again later.

Since vSphere 7.0, clustered VMDKs on VMFS-6 datastores (with eager-zeroed thick disks) are an alternative to pRDMs for WSFC, which simplifies management. NPIV, which gives a VM its own Fibre Channel WWPNs on an RDM, is being retired: the vSphere 8.0 release notes announce its deprecation in a future release.

NVMe and NVMe over Fabrics

NVMe is a storage protocol designed for flash, with deep parallel queues. vSphere uses it in three places:

• Local NVMe devices: PCIe SSDs in the host, often used for vSAN (ESA requires NVMe) or local VMFS.
• NVMe over Fabrics (NVMe-oF): NVMe commands carried over a network to an external array.
• Virtual NVMe controller: a VM-side controller type (hardware version 13 and later) that lowers guest I/O overhead. It is independent of the physical transport.

ESXi's NVMe-oF transports:

Key points:

• NVMe devices present namespaces rather than LUNs, and you connect to array controllers through discovery.
• ESXi claims NVMe devices with the VMware High-Performance Plug-in (HPP) instead of the Native Multipathing Plug-in used for SCSI. HPP has its own path selection schemes, such as load balancing by round robin or by latency.
• Use cases: latency-sensitive databases and analytics on all-flash arrays, or moving off FC SCSI to NVMe without changing the fabric (FC-NVMe), or using existing Ethernet (NVMe/TCP).

iSER: iSCSI Extensions for RDMA

iSER keeps the iSCSI model (targets, IQNs, port binding) but moves the data transfer onto RDMA, using RDMA-capable NICs (RoCE). This cuts the host CPU work and latency of software iSCSI.

• Enable the adapter from the ESXi shell with esxcli rdma iser add, which creates a VMware iSER adapter (a vmhba).
• Bind it to a VMkernel adapter on an RDMA-capable NIC, then configure discovery and targets as with iSCSI.
• Use case: an iSCSI array that supports iSER, where you want lower latency and CPU cost without moving to a new protocol.

Persistent Memory (PMem)

PMem (Intel Optane persistent memory) sits between DRAM and flash: byte-addressable like memory, and persistent like storage. vSphere 8 supports it in two ways:

PMem can also run in Memory Mode, where it transparently expands host memory capacity.

• Use cases: in-memory databases and applications that need very fast restarts or persistent in-memory structures.
• Constraints: PMem is local to each host, so VMs using it need destination hosts with PMem for vMotion, and HA restarts need spare PMem.
• Status: Intel discontinued Optane, and Broadcom ends PMem support starting with VMware Cloud Foundation 9.0. It is still in the vSphere 8 blueprint, so know the modes.

Choosing Among Them

Exam Traps

• pRDM means no VM snapshots. Backup tools that rely on VM snapshots cannot protect a physical-mode RDM, so plan array-level protection.
• NVMe devices are not claimed by NMP. If a question asks which plug-in manages NVMe paths, the answer is the High-Performance Plug-in (HPP).
• NVMe/TCP needs no special NIC; NVMe/RDMA and iSER do. Match the transport to the hardware in the question.
• PMem is host-local. Plan vMotion and HA capacity accordingly, and remember that support ends with VMware Cloud Foundation 9.0.