Virtualisation has become the watchword of the IT industry. As senior executives look to exploit their existing investments, the idea that servers or storage might only see utilisation rates of 15-20% has become unpalatable. And as businesses continue to invest heavily in storage systems, virtualisation is being touted as one of the main means of driving up utilisation rates.

While much of the virtualisation focus has been on servers, virtualisation of storage – and of SANs in particular – has attracted the interest of many IT decision makers looking to get the most out of their existing assets and to reduce the need for future investment.

And virtualisation technologies are not just being used to push up utilisation rates: it is also transforming the way businesses are managing their data, becoming a cornerstone of information lifecycle management (ILM) strategies, dealing with peaks and troughs in demand, increasing security and improving business continuity.

“Virtualisation is a way to deal with the day to day problems of storage by getting rid of its ‘storageness’,” says Steve Legg, IBM’s storage virtualisation architect. “Organisations are looking for capacity and performance, not the behaviours associated with vendor ‘X’ – its ‘storageness’.”

When a new project begins in any organisation, that project will need a certain amount of storage. Often, much of the storage administrator’s task    will be finding space on different arrays for the project – or recommending the organisation buy new storage. “Many people say to me, ‘Much of my job is putting little bits of storage together to make big storage’,” says Legg. This is where virtualisation can really help.

Virtualisation layers

Storage virtualisation comes in many forms. There are routers and controllers from companies such as Cisco and Brocade that are capable of making a storage array or several fabrics appear as many different volumes, allowing organisations to isolate particular applications to particular virtual volumes. This can improve security, since different applications can no longer access the same volume, and reduce the chance of unexpected interactions between applications.

It is also possible to partition SAN fabrics – the hardware that connects servers to the storage devices – isolating problems only to ‘virtual fabrics’ when they occur and reducing operational load. However, James Opfer, research vice president, Gartner Dataquest Research, says the uptake of such devices is not that high so their use is uncommon, despite having been on the market for several years.

Instead, virtualisation usually involves uniting rather than dividing SANs and SAN fabrics. This can be done at many possible layers of the SAN, each of which has its own benefits and drawbacks.

At the server level, it is possible to install software that creates a virtual view of the SAN storage available. However, this approach can have performance overheads, and may be slower than hardware-based systems. It also only works for individual servers, and cannot accommodate multiple separate operating systems.

Virtualisation is also possible at the storage level by building it into the storage arrays. This form of virtualisation is faster than virtualisation at the server-level but not as capable at virtualising across SAN fabrics.

“Virtualisation should exist in the brain of the storage system, not anywhere else,” argues John Joseph, VP of marketing at EqualLogic. His company, which sells iSCSI-based SAN hardware, has a patented virtualisation technology included in each array. When an additional array is connected to the fabric, the virtualisation system is able to spot the other arrays on the fabric and, depending on the settings, reorganise stored data, potentially migrating data from one array to another if it is nearly full. If the new array has different capabilities, the arrays can negotiate to move the more frequently requested or more important data onto the faster or more powerful array, for instance.

However, virtualisation on the array tends to require proprietary systems, and does not work when arrays are on different fabrics, and in the majority of cases, does not work with arrays from different manufacturers.

Fabric or switch-level virtualisation is garnering the most interest since it is possible to overcome most of these problems, even if it does require expensive hardware. One of the key concerns for customers is that they are able to virtualise and migrate data between different types of storage, and between different vendors’ products.

Virtualise everything

Virtualisation can either be done ‘inband’ or ‘out-of-band’. Inband requires hardware that can sit between all the network’s storage arrays and servers and mediate data transfer between them using standard SCSI commands. If the hardware cannot move data at the same speed as the rest of the SAN, this can affect scalability and throughput, so the requisite hardware has to contain considerable processing power, pushing up costs.

Out-of-band, however, requires agents to be installed so is more rarely used, although EMC, for example, offers an out-of-band virtualisation appliance called InVista.

As well as pooling a set of arrays into a single resource, virtualisation can also provide a number of other capabilities, depending on the systems involved. “When you’ve 10 projects and want to allocate 1TB to each project, you can virtualise everything instead,” says NetApp’s consultant systems engineer, Dave Logan “You present your storage as 10TB when you only have 3-4TB at the back end, say, and then use a policy engine to expand volumes and delete snapshots accordingly.”

Virtualisation can also provide ILM capabilities using policy engines. By setting up rules for data importance, it’s possible for the virtualisation system to migrate data from one tier of storage to another or even archiving systems, while presenting the pooled storage as a single resource. The data will appear to be in the same location as before, but will actually be on a completely different and cheaper storage system.

Alternatively, if an operator knows that certain data is accessed more frequently on certain days, it is possible to automatically move data between different tiers of storage to cope with the different levels of demand.

Of equal interest to many organisations is the possibility not just of getting more out of their existing storage but for seamless back-up to other systems using disk-to-disk replication. Instead of migrating data, the virtualisation system can copy data to other SANs or storage media while moving data on the primary SAN.

While all these technologies can bring additional business benefits to SANs, one of the bigger knock-on effects is cost savings – not in technology acquisition but in management costs and staffing levels. “There is no question that for some customers, cost has been a biggest barrier to adoption,” says Dell’s enterprise marketing manager, Hugh Jenkins. “We have been able to take the cost down considerably over the last three years. But complexity has remained the big barrier.” By making storage easier to manage and to automate, much of those costs can be reduced.

 Storage virtualisation is by no means a panacea and implementations can be extremely complex. But done properly, it provides a way for organisations not only to get more from what they have already got, but to make managing it simpler and more cost effective.

Further reading

• More articles from Information Age's Storage Business Briefing.
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