ESX Configuration Guide

Various management products are best run centrally (not on the ESX machine), while others run well on the service console or on the virtual machines. VMware does not certify these applications or provide a compatibility matrix. To find out whether a SAN management application is supported in an ESX environment, contact the SAN management software provider.

Limitations of Raw Device Mapping

Certain limitations exist when you use RDMs.

nNot available for block devices or certain RAID devices – RDM uses a SCSI serial number to identify the mapped device. Because block devices and some direct-attach RAID devices do not export serial numbers, they cannot be used with RDMs.

nAvailable with VMFS-2 and VMFS-3 volumes only – RDM requires the VMFS-2 or VMFS-3 format. In ESX, the VMFS-2 file system is read only. Upgrade it to VMFS-3 to use the files that VMFS-2 stores.

nNo snapshots in physical compatibility mode – If you are using an RDM in physical compatibility mode, you cannot use a snapshot with the disk. Physical compatibility mode allows the virtual machine to manage its own snapshot or mirroring operations.

Snapshots are available in virtual mode.

nNo partition mapping – RDM requires the mapped device to be a whole LUN. Mapping to a partition is not supported.

Raw Device Mapping Characteristics

An RDM is a special mapping file in a VMFS volume that manages metadata for its mapped device. The mapping file is presented to the management software as an ordinary disk file, available for the usual filesystem operations. To the virtual machine, the storage virtualization layer presents the mapped device as a virtual SCSI device.

Key contents of the metadata in the mapping file include the location of the mapped device (name resolution), the locking state of the mapped device, permissions, and so on.

RDM Virtual and Physical Compatibility Modes

You can use RDMs in virtual compatibility or physical compatibility modes. Virtual mode specifies full virtualization of the mapped device. Physical mode specifies minimal SCSI virtualization of the mapped device, allowing the greatest flexibility for SAN management software.

In virtual mode, the VMkernel sends only READ and WRITE to the mapped device. The mapped device appears to the guest operating system exactly the same as a virtual disk file in a VMFS volume. The real hardware characteristics are hidden. If you are using a raw disk in virtual mode, you can realize the benefits of VMFS such as advanced file locking for data protection and snapshots for streamlining development processes. Virtual mode is also more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk file.

In physical mode, the VMkernel passes all SCSI commands to the device, with one exception: the REPORT LUNs command is virtualized so that the VMkernel can isolate the LUN to the owning virtual machine. Otherwise, all physical characteristics of the underlying hardware are exposed. Physical mode is useful to run SAN management agents or other SCSI target-based software in the virtual machine. Physical mode also allows virtual-to-physical clustering for cost-effective high availability.

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Chapter 10 Raw Device Mapping

Dynamic Name Resolution

The RDM file supports dynamic name resolution when a path to a raw device changes.

VMFS uniquely identifies all mapped storage devices, and the identification is stored in its internal data structures. Any change in the path to a raw device, such as a Fibre Channel switch failure or the addition of a new HBA, can change the device name. Dynamic name resolution resolves these changes and automatically associates the original device with its new name.

Raw Device Mapping with Virtual Machine Clusters

Use an RDM with virtual machine clusters that need to access the same raw LUN for failover scenarios. The setup is similar to that of a virtual machine cluster that accesses the same virtual disk file, but an RDM replaces the virtual disk file.

Figure 10-3. Access from Clustered Virtual Machines

Host 3

Host 4

VM3		VM4
	“shared” access
	mapping file	address	mapped
		resolutiion
			device
	VMFS volume

Comparing Available SCSI Device Access Modes

The ways of accessing a SCSI-based storage device include a virtual disk file on a VMFS datastore, virtual mode RDM, and physical mode RDM.

To help you choose among the available access modes for SCSI devices, Table 10-1 provides a quick comparison of features available with the different modes.

Table 10-1. Features Available with Virtual Disks and Raw Device Mappings

ESX Features	Virtual Disk File	Virtual Mode RDM	Physical Mode RDM
SCSI Commands Passed	No	No	Yes
Through			REPORT LUNs is not passed
			through
vCenter Server Support	Yes	Yes	Yes
Snapshots	Yes	Yes	No
Distributed Locking	Yes	Yes	Yes
Clustering	Cluster-in-a-box only	Cluster-in-a-box and	Physical to Virtual Clustering
		cluster-across-boxes
SCSI Target-Based Software	No	No	Yes

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