In the thick of a thin-provisioning debate

At first sight, thin provisioning is not a subject likely to lead to discord. But it can, as journalists and members of HP's management team discovered at the recent StorageWorks conference in Las Vegas.

Thin provisioning or, as HP calls it, Dynamic Capacity Management (DCM), is straightforward to understand and potentially far-reaching in its implications.

Companies that use thin provisioning include Hitachi, 3PAR and VMware, which has thin provisioning in some of its client software. How does it work? The principles are simple enough.

When an IT manager first sets up a system, he or she will provision the servers, which means deciding how much disk space is likely to be needed for storage and so on.

If we say the manager is starting a new company with 100 staff who all need computer systems, the IT department will set up the servers to run the systems and allocate each member of staff with passwords, set-up codes and so on, and each will have an email address with their own PC and email system.

Each user is allocated a certain amount of space on the server for their emails and the other applications they need, and this may be 10GB to each standard user, 100GB to each IT manager and senior member of staff and more for sys-ops.

All the storage is allocated immediately, even though users will only fill up space over time. The result is that all servers are over-provisioned. Estimates vary on the extent to which they are over-provisioned, but Gartner and others reckon that around 50 percent of disk space is unused at any one time.

The result is that a lot of disks are spinning, and therefore using electricity, for no good reason. It is wasteful in resource and costs money — and thin provisioning claims to be the answer. With thin provisioning, you only provision what you are likely to need. If you need more space at some point then you start up another drive, but in the meantime you can leave it switched off.

The controversy was started by Mark Gonzalez, vice president of enterprise storage and servers sales, HP America. He ran through HP's offering and highlighted DCM, HP's own thin provisioning. The StorageWorks EVA4100, 6100 and 8100 midrange disk arrays offer DCM, which HP says is "similar to thin provisioning". DCM, the company's press statement at the time continued, "enables customers to double capacity utilisation rates and delay the purchase of additional hard drives". It does this by "leveraging the new virtual disk service (VDS) volume shrink feature in Microsoft Windows Server 2008". It is basically adapting a standard Microsoft feature, and so it should work. But does it work as thin provisioning does, by fooling the operating system into believing that it has more space when really it does not?

"DCM continuously monitors storage utilisation rates and automatically grows or shrinks host volumes to match application data needs," HP went on. So that explains it. Instead of fooling the software into thinking the storage is available when it has been allocated somewhere else, the HP solution is to just shrink the file sizes away until the storage is needed.

The purists argue that this is not thin provisioning. Computerwire's Tim Stammers asked Gonzalez what the difference was between this form of thin provisioning and that offered by the high-end suppliers such as 3PAR or Network Appliances. "Ours works," he said. But according to ESG analyst Tony Asaro, DCM is not thin provisioning. "If I allocate 2TB to a volume, it will take 2TB," he noted.

So is HP's solution thin provisioning or not? Some might say it is an academic argument: if thin provisioning is about saving on disk space, thus running systems efficiently and in a more "green" way by cutting down on power use, and both methods achieve this, then what is the problem?

Well, it is not an academic argument to some. Hitachi, for example, can claim that it has have one of the best solutions for thin provisioning and that it was one of the first on the market with one after 3PAR. The company can also claim that on the argument about which form of thin provisioning is best they are neutral, more or less, and will have to stay that way.

Hitachi is a long-term partner with HP and, while HP may be selling the EVA4100 series in competition with Hitachi's own, home-grown thin-provisioning solution, HP sells the Hitachi solution as well. The HP XP21000 is HP's other thin-provisioning solution, which it started selling last month and is 100 percent Hitachi. And that one is purist-approved thin provisioning.