vSphere Storage - ESXi 5.1 - Documentation - VMware

Storage Thin Provisioning 22
With ESXi, you can use two models of thin provisioning, array-level and virtual disk-level.
Thin provisioning is a method that optimizes storage utilization by allocating storage space in a flexible ondemand
manner. Thin provisioning contrasts with the traditional model, called thick provisioning. With thick
provisioning, large amount of storage space is provided in advance in anticipation of future storage needs.
However, the space might remain unused causing underutilization of storage capacity.
The VMware thin provisioning features help you eliminate storage underutilization problems at the datastore
and storage array level.
This chapter includes the following topics:
n “Storage Over-Subscription,” on page 225
n “Virtual Disk Thin Provisioning,” on page 225
n “Array Thin Provisioning and VMFS Datastores,” on page 230
Storage Over-Subscription
Thin provisioning allows you to report more virtual storage space than there is real physical capacity. This
discrepancy can lead to storage over-subscription, also called over-provisioning.
When you use thin provisioning, you should monitor actual storage usage to avoid conditions when you run
out of physical storage space.
Virtual Disk Thin Provisioning
When you create a virtual machine, a certain amount of storage space on a datastore is provisioned to virtual
disk files.
By default, ESXi offers a traditional storage provisioning method for virtual machines. With this method, you
first estimate how much storage the virtual machine will need for its entire life cycle. You then provision a
fixed amount of storage space to its virtual disk in advance, for example, 40GB, and have the entire provisioned
space committed to the virtual disk. A virtual disk that immediately occupies the entire provisioned space is
a thick disk.
ESXi supports thin provisioning for virtual disks. With the disk-level thin provisioning feature, you can create
virtual disks in a thin format. For a thin virtual disk, ESXi provisions the entire space required for the disk’s
current and future activities, for example 40GB. However, the thin disk uses only as much storage space as the
disk needs for its initial operations. In this example, the thin-provisioned disk occupies only 20GB of storage.
As the disk requires more space, it can grow into its entire 40GB provisioned space.
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