Thin provisioning
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Thin provisioning[1] 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.
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[edit] Overview
First introduced by 3PAR Inc. in 2002, Thin Provisioning, in a shared storage environment, is the allocation of blocks as data is written real-time versus the traditional method of allocating all the blocks up front. This methodology eliminates almost all whitespace which helps avoid the poor utilization rates, often as low as 10%, that occur in the traditional storage allocation method where large pools of storage capacity are allocated to individual servers but remain unused (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% with very little 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.
Previous systems generally required large amounts of storage to be physically pre-allocated because of the complexity and impact of growing volume (LUN) space. Thin provisioning enables over-allocation or over-subscription.
[edit] Over-allocation
Over-allocation or over-subscription is a mechanism that allows server to view more storage capacity than has been physically reserved on the storage array itself. This allows flexibility in growth of storage volumes, without having to predict accurately how much a volume will grow. Instead, block growth becomes sequential. 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. The servers, and by extension the applications that reside on them, view a full size volume from the storage but the storage itself only allocates the blocks of data when they are written.
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 server and residing application(s).
The Over-allocation concept was first introduced through DataCore Software's SANsymphony 5.0 product in 2001.
[edit] Notes
- ^ Thin provisioning is called sparse volumes in some contexts.
