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In computing, thin provisioning involves 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 layer 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 acts as if it has real memory allocated. The sum of the allocated virtual memory assigned to tasks typically exceeds the total of real memory.
The efficiency of thin or thick/fat provisioning is a function of the use case, not of the technology. Thick provisioning is typically more efficient when the amount of resource used very closely approximates to the amount of resource allocated. Thin provisioning offers more efficiency 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.
Just-in-time allocation differs from thin provisioning. Most file systems back files just-in-time but are not thin provisioned. Overallocation also differs from thin provisioning; resources can be over-allocated / oversubscribed without using virtualization technology, for example overselling seats on a flight without allocating actual seats at time of sale, avoiding having each consumer having a claim on a specific seat number.
Thin provisioning is a mechanism that applies to large-scale centralized computer disk-storage systems, SANs, and storage virtualization systems. Thin provisioning allows space to be easily allocated to servers, on a just-enough and just-in-time basis. Thin provisioning is called "sparse volumes" in some contexts.