6.5.1

For virtual machines running on an ESXi host, you can store virtual machine data directly on a SAN LUN
instead of using a virtual disk file. This option is useful if in your virtual machines you run applications that
must detect the physical characteristics of the storage device. Mapping a SAN LUN also allows you to
use existing SAN commands to manage storage for the disk.
To accelerate virtual machine performance, you can configure virtual machines to use vSphere Flash
Read Cache™. For details about Flash Read Cache behavior, see the vSphere Storage documentation.
When you map a LUN to a VMFS volume, vCenter Server or the ESXi host creates a raw device mapping
(RDM) file that points to the raw LUN. Encapsulating disk information in a file allows vCenter Server or the
ESXi host to lock the LUN so that only one virtual machine can write to it. This file has a .vmdk extension,
but the file contains only disk information that describes the mapping to the LUN on the 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 store only its data in a virtual disk file.
The amount of free space in the datastore is always changing. Ensure that you leave sufficient space for
virtual machine creation and other virtual machine operations, such as growth of sparse files, snapshots,
and so on. To review space utilization for the datastore by file type, see the vSphere Monitoring and
Performance documentation.
Thin provisioning lets you create sparse files with blocks that are allocated upon first access, which allows
the datastore to be over-provisioned. The sparse files can continue growing and fill the datastore. If the
datastore runs out of disk space while the virtual machine is running, it can cause the virtual machine to
stop functioning.
About Virtual Disk Provisioning Policies
When you perform certain virtual machine management operations, you can specify a provisioning policy
for the virtual disk file. The operations include creating a virtual disk, cloning a virtual machine to a
template, or migrating a virtual machine.
NFS datastores with Hardware Acceleration and VMFS datastores support the following disk provisioning
policies. On NFS datastores that do not support Hardware Acceleration, only thin format is available.
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