vsphere-esxi-vcenter-server-55-storage-guide

Storage Thin Provisioning 25With 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 ondemandmanner. Thin provisioning contrasts with the traditional model, called thick provisioning. Withthick provisioning, large amount of storage space is provided in advance in anticipation of future storageneeds. However, the space might remain unused causing underutilization of storage capacity.The VMware thin provisioning features help you eliminate storage underutilization problems at thedatastore and storage array level.This chapter includes the following topics:• “Storage Over-Subscription,” on page 245• “Virtual Disk Thin Provisioning,” on page 245• “Array Thin Provisioning and VMFS Datastores,” on page 249Storage Over-SubscriptionThin provisioning allows you to report more virtual storage space than there is real physical capacity. Thisdiscrepancy 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 yourun out of physical storage space.Virtual Disk Thin ProvisioningWhen you create a virtual machine, a certain amount of storage space on a datastore is provisioned tovirtual 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 thenprovision a fixed amount of storage space to its virtual disk in advance, for example, 40GB, and have theentire provisioned space committed to the virtual disk. A virtual disk that immediately occupies the entireprovisioned space is a thick disk.ESXi supports thin provisioning for virtual disks. With the disk-level thin provisioning feature, you cancreate virtual disks in a thin format. For a thin virtual disk, ESXi provisions the entire space required for thedisk’s current and future activities, for example 40GB. However, the thin disk uses only as much storagespace as the disk needs for its initial operations. In this example, the thin-provisioned disk occupies only20GB of storage. As the disk requires more space, it can grow into its entire 40GB provisioned space.VMware, Inc. 245