A virtual disk stores the actual VM data and the VM can be configured to use a new disk, attach an existing disk, or map a SAN LUN. A LUN to a VMFS map is referred to as a Raw Device Mapping (RDM) that points to the raw LUN. In this case, the .vmdk file (.vmdk files will be discussed later in the file structure) doesn't store data, as the data is stored in the LUN, but it contains the mapping to the LUN disk information.

Virtual disks can be moved across different data stores connected to the host on which the VM runs. When a new disk is created, it can be provisioned in three different formats, depending on the requirements:

• Thick provision lazy zeroed: This is the default format; space on the datastore is allocated when the VM is created, and data on the physical device is not erased.
• Thick provision eager zeroed: This is the format used to support specific configurations, such as vSphere FT or some SQL installations; it allocates space on the datastore when the disk is created. Compared to the lazy zeroed format, data on the physical device is zeroed out at creation time. The thick provision eager zeroed format takes longer to be provisioned.

• Thin provision: This is used to save space; it's the fastest method to create a new disk. This format doesn't allocate all the requested disk space upon creation. At first, it only uses the space required by the initial operations of the disk, growing in size until the maximum configured size is reached.

The following screenshot shows the different Disk Provisioning types:

Each virtual disk has a disk mode, as follows:

• Dependent: Dependent disks are included in snapshots. Snapshots will be discussed in the Managing VMs section.
• Independent-Persistent: Independent disks act the same as dependent disks, but the writes are committed to the disk immediately and the disks are not affected by the snapshots. Even if you create a snapshot, the data will be directly written to the disk.
• Independent-Nonpersistent: Any writes made to non-persistent disks are discarded when you power off or reset the virtual machine.

Virtual disks can only be connected to a single SCSI controller. You can connect an existing virtual disk to a different SCSI controller. For example, once the VM tools are installed, you can assign a new PVSCSI controller, as shown in the following screenshot: