Thin provisioning (TP) is a method of optimizing the efficiency with which the available space is utilized in storage area networks (SAN). TP operates by allocating disk storage space in a flexible manner among multiple users, based on the minimum space required by each user at any given time.

In the conventional storage provisioning model, also known as fat provisioning (FP), storage space is allocated beyond current needs, in anticipation of growing need and increased data complexity. As a result, the utilization rate is low. Large amounts of storage space are paid for but may never be used. In TP, these problems are eliminated while keeping overhead low. Added benefits include reduced consumption of electrical energy, smaller hardware space requirements and reduced heat generation compared with traditional networked storage systems.

Wikipedia Says:

Thin provisioning is the act of using virtualization technology to give the appearance of having more physical resources than are actually available. If a system always has enough resource to simultaneously support all of the virtualized resources, then it is not thin provisioned. The term thin provisioning is applied to disk later in this article, but could refer to an allocation scheme for any resource. For example, real memory in a computer is typically thin provisioned to running tasks with some form of address translation technology doing the virtualization. Each task believes that it has real memory allocated. The sum of the allocated virtual memory assigned to tasks is typically greater than the total of real memory.

The efficiency of thin or thick/fat provisioning is a function of the use case, not the technology. Thick provisioning is typically more efficient when the amount of resource used is very close to the amount of resource allocated. Thin provisioning is more efficient where the amount of resource used is much smaller than allocated so that the benefit of providing only the resource needed exceeds the cost of the virtualization technology used.

Other Definition:

In data storage terminology, thin provisioning is the term used to describe the consolidation and automated process of allocating just “the exact required amount” of server space at the time it is required. Thin provisioning is most commonly used in centralized large storage systems such as SANs and also in storage virtualization environments where administrators plan for both current and future storage requirements and often over-purchase capacity, which can result in wasted storage. Since thin provisioning is designed to allocate exactly what is needed, exactly when it is needed, it removes the element of “paid for but wasted” storage capacity. Additionally, as more storage is needed additional volumes can be attached to the existing consolidated storage system.

Thin Provisioining in XenServer:

Thin provisioning is a method by which the hypervisor can virtually allocate all of the storage assigned to a virtual machine while only physically allocating the space that is actually required by the virtual machine. This can save tremendous amounts of space.