VMware 4817V62 - vSphere - PC, Administration Manual

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Create Raw Device Mappings
For virtual machines running on an ESX/ESXi host, instead of storing virtual machine data in a virtual disk
file, you can store the data directly on a SAN LUN. This is useful if you are running applications in your virtual
machines that must know the physical characteristics of the storage device. Additionally, mapping a SAN LUN
allows you to use existing SAN commands to manage storage for the disk.
When you map a LUN to a VMFS volume, vCenter Server creates a file that points to the raw LUN.
Encapsulating disk information in a file allows vCenter Server to lock the LUN so that only one virtual machine
can write to it.
N
OTE
This file has a
.vmdk
extension, but the file contains only disk information describing the mapping to
the LUN on the ESX/ESXi system. The actual data is stored on the LUN.
You cannot deploy a virtual machine from a template and store its data on a LUN. You can only store its data
in a virtual disk file.
Procedure
1 Select a target LUN.
2 Select whether you want to store the LUN mapping file on the same datastore as the virtual machine files,
or whether you want to store them on a separate datastore.
3 Select a datastore.
4 Select a compatibility mode.
5 (Optional) Configure advanced options by selecting a virtual device node.
Virtual Disk Compatibility Modes
Virtual disk compatibility modes provide flexibility in how Raw Device Mappings (RDM) function.
Virtual Compatibility Mode
Virtual mode for an RDM specifies full virtualization of the mapped device. It appears to the guest operating
system exactly the same as a virtual disk file in a VMFS volume. The real hardware characteristics are hidden.
Virtual mode enables you to use VMFS features such as advanced file locking and snapshots. Virtual mode is
also more portable across storage hardware than physical mode, presenting the same behavior as a virtual disk
file. When you clone the disk, make a template out of it, or migrate it (if the migration involves copying the
disk), the contents of the LUN are copied into a virtual disk (
.vmdk
) file.
Physical Compatibility Mode
Physical mode for the RDM specifies minimal SCSI virtualization of the mapped device, allowing the greatest
flexibility for SAN management software. 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 for 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. A
LUN configured for physical compatibility cannot be cloned, made into a template, or migrated if the migration
involves copying the disk.
Do Not Create a Disk
When you create a virtual machine, you can select not to create a virtual disk.
Select this option if you want to create a virtual machine without a disk, or if you want to add disks to the
virtual machine later using the Virtual Machine Properties dialog box.
Chapter 11 Creating Virtual Machines
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