Array-based Thin Provisioning over VMware's Thin Provisioning

Over the last few years thin provisioning has steadily moved into the main stream of storage management - so much so that not only has it found its way onto many leading storage systems but into operating systems as well. Clearly one of the largest endorsements of using thin provisioning at the operating system level came last year when VMware announced its inclusion of thin provisioning as an option within vSphere 4.

But with thin provisioning now available at both the OS and storage system levels, organizations need to quantify at which of these two levels that they can derive the greatest number of benefits from thin provisioning or if, in fact, there are sufficient benefits at both levels to implement both storage and VMware thin provisioning .

A place that is as good as any to start in trying to understand the benefits for each is a video that was embedded in a blog on the StorageRap website on September 1, 2009. This video contrasts the benefits of using VMware's thin provisioning with using the thin provisioning feature found on the 3PAR InServ Storage Server.

The initial scenario that the presenter, 3PAR's Michael Haag, illustrates is when the storage allocated to VMware is a fat volume (i.e. NOT thinly provisioned) from traditional storage. All of this storage capacity is discovered and allocated by the VMware file system (VMFS) and brought under its management.

It is in this scenario that the primary value of VMware's new thin provisioning feature comes into play. As new virtual machines and their associated VMDK files are created, VMFS will only allocate as much physical space as each individual VMDK file actually needs, reserving only enough space for the VMDK's data. This results in storage savings and enables vSphere to more efficiently meet the storage requirements of the virtual machines it hosts as well as open the door to hosting more VMs.

However there is still a very real up front storage cost with VMware's thin provisioning implementation: all VMFS capacity must be provisioned (physically) up front instead of as it is needed for each VM and VMDK.

So while VMware vSphere's thin provisioning feature certainly helps organizations get more value from "fat" storage with a thin methodology from VMFS to the VM, it still leaves the door open to increase storage utilization with array-based thin provisioning. Array-based thin provisioning not only associates physical capacity with the VMDK only as writes occur, but does so with VMFS as well so there is no up front allocation or waste.

Consider this scenario that occurs every day in organizations of all sizes. It is only natural for an application administrator to request more storage capacity than the application needs. In this case, VMware's thin provisioning helps alleviate this over provisioning request by enabling storage administrators to thinly provision VMFS' allocated storage capacity to the VM-based application as is written.

However the storage administrator is still forced to hedge his bets. While he may suspect the application administer has over estimated how much storage capacity he needs, the storage administrator needs to over provision the networked storage assigned to that vSphere server to account for the situations where the application owner's estimates are correct or even too low.

It is for this reason that the presenter in this video as well as other virtualization experts not affiliated with 3PAR recommend, at a minimum, using thin provisioning at the array level. VMware thin provisioning can be used in conjunction with array-based thin provisioning; however, thin provisioning at the array level should be used at minimum because it provides the most efficient use of storage capacity for three reasons:
  • Storage capacity is only allocated and consumed (to VMFS and the VMDK) when data is actually written by the application on the VM to the attached storage system.
  • Storage capacity does not need to be over-provisioned
  • Storage management can remain a function of the storage team for those enterprise organizations that have this separation of duties. However because of 3PAR's integration with VMware's vCenter, organizations can also monitor, manage and report on storage through this management console.
Finally there are cases where it makes sense to use the two thin provisioning solutions together. The additional overhead of adding VMware TP on top of array-based TP can be beneficial in cases where separate Server and Storage Administrators want the flexibility to over-provision at their respective layer.

The growing acceptance of thin provisioning in the data center is leading to increased cost savings and efficiencies but, as this example with VMware vSphere illustrates, sometimes it is not always clear which is the best way to implement it.
The short answer is that if the storage system you are using now does not offer thin provisioning, using vSphere's thin provisioning feature is clearly the way to go. But in circumstances where you have both VMware and a storage system from 3PAR available to you and if thin provisioning can only be used in one place, then it should absolutely be done at the array-based layer to maximize utilization and minimize upfront costs.
Administrators can then decide to deploy VMware thin provisioning on top of 3PAR thin provisioning if it provides enough management benefits to the VMware administrator to outweigh the need to manage thin provisioning in two places. 


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