Vendor claims about storage virtualisation are seriously flawed

IT managers have been advised to be wary of vendor hype surrounding storage virtualisation as it is a technology that is poorly defined, misunderstood and not widely used, according to Dr Kevin McIsaac, an advisor at research firm IBRS.

Despite all the hype, McIsaac said over the next two years network based storage virtualisation will remain a niche, while thin provisioning will enjoy rapid adoption in the enterprise.

And while McIsaac readily admits server virtualisation is one of the best IT infrastructure trends to emerge in many years, he said the situation is very different when it comes to storage virtualisation.

"This idea of being able to layer virtualisation over existing storage arrays is seriously flawed," he warned.

McIsaac said a reasonable definition of storage virtualisation is "the abstraction of logical storage from physical storage". However, given the sweeping nature of this definition it is not surprising that the technology creates confusion.

"The first step in understanding storage virtualisation is to recognise that many of today's commonly used techniques and technologies are examples of virtualisation including a filesystem or a storage array," McIsaac said.

Rather than thinking of it as a specific new product or feature, McIsaac said it should be thought of as a broad technique that can be deployed at any layer of the storage hardware and software stack to simplify the storage environment.

"Network based virtualisation, which involves using a device in the network to provide an abstraction layer over storage arrays, is usually what vendors mean when they refer to storage virtualisation," he explained.

"The idea is to layer virtualisation over existing arrays to create a single storage pool, simplify management and eliminate vendor lock-in. But this idea has significant flaws."

Organisations typically moved to an external storage array, either via a SAN or a NAS, to achieve higher utilisation by sharing the same pool of spare disk across multiple servers.

McIsaac said applying network based storage virtualisation to pool arrays sn't likely to improve this environment if efficient utilisation hasn't already been achieved.

"Network based storage virtualisation results in a lowest common denominator view of the infrastructure, eliminating the value added features of the array; this investment in the advanced features of the storage array could be lost making it a waste of money," he said.

"Also the addition of the virtualisation layer adds yet more complexity to the environment, it can introduce a performance bottleneck and add yet another potential.source of failure. And while it may eliminate vendor lock-in at the storage array it replaces it with lock-in at the virtualisation layer."

McIsaac believes array based virtualisation is the best solution and the next major step forward is thin provisioning. This is a provisioning mechanism for allocating storage capacity on a just-in-time basis from a single shared capacity pool. In this approach the physical storage is only allocated when it is used, not when it is provisioned.

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"Administrators typically over allocate storage because of the complexity, impact and additional work to grow capacity. For example, if an application requires 50Gb the DBA will request 100Gb for head room, then the storage administrator doubles that just to be sure and provisions 200Gb," he said.

"Thin provisioning eliminates unused storage, reduces capital costs and simplifies capacity planning. While it isn't widely used at present its adoption will accelerate rapidly as vendor support widens and administrators become aware of its benefits.

"Im predicting that over the next 18 months thin provisioning will become as pervasive as other array-based virtualisation features such as RAID and snapshots."

McIsaac advised administrators to review current vendors and technologies recognising that "some vendor lock-in is unavoidable".

He said avoid using network based storage virtualisation, instead minimising the number of array vendors deployed in the SAN or NAS.

ARD Consulting IT manager, Eric Biggsley, said when it comes to storage its all about simplicity which is why he opted for an iSCSI SAN.

"Also I was making my purchasing decision when VMWare announced it would be adding iSCSI support," he said.

"This was great news because a SAN is necessary to get the most out of virtualisation, but I didn't want to do it with Fibre Channel because it was just too costly and we didn't really have the resources to support it."

Since then Biggsley has combined server virtualisation with SAN technology.

"Virtualization was one of the key drivers behind the selection of an iSCSI SAN," he said.