Thin provisioning

Thin provisioning [Thin provisioning is called sparse volumes in some contexts.] 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.

Overview

In a storage consolidation environment, where many applications are sharing access to the same storage array, thin provisioning allows administrators to maintain a single free space buffer pool to service the data growth requirements of all applications. This avoids the poor utilization rates, often as low as 10%, that occur on traditional storage arrays where large pools of storage capacity are allocated to individual applications, but remain unused (i.e. not written to). This traditional model is often called fat provisioning.

With thin provisioning, storage capacity utilization efficiency can be automatically driven up towards 100%, without heavy administrative overhead. Organizations can purchase less storage capacity up front, defer storage capacity upgrades in line with actual business usage, and save the operating costs (electricity and floorspace) associated with keeping unused disk capacity spinning.

Previously systems generally required large amounts of storage to be physically pre-allocated because of the complexity and impact of growing volume space. Thin provisioning enables over-allocation or over-subscription. The underlying concept of virtualization dates to the 1960s, with IBM's VM operating system.

Over-allocation

Over-allocation or over-subscription is a mechanism that allows server applications to be allocated more storage capacity than has been physically reserved on the storage array itself. This allows flexibility in growth and shrinkage of application storage volumes, without having to predict accurately how much a volume will grow or contract. Physical storage capacity on the array is only dedicated when data is actually written by the application, not when the storage volume is initially allocated.

Some metaphors for over-subscription: 1) Use of a credit card or 2) Playing the "float" on a checking account.

As a practical consideration, a storage manager needs to monitor actual storage used, adding additional storage capacity such as disks, tapes, solid-state drives (SSD), etc. as necessary to satisfy the write requests of the application(s).

Notes

ee also

*Storage Virtualization