EMC's Invista product is an example of an "out of band" solution, as is IBM's SAN Volume Controller. i.e. both sit on the network and control data movement through network switches. HDS's Tagmastore is "in band" - which means all the data passes directly through the device; usually a storage array.

There are arguments for and against each, most commonly relating to their relative performance, reliability, resilience, and ability to manage heterogeneous environments in multiple datacentres.

The lines between these "in band" and "out of band" options are however, becoming increasingly blurred as "intelligent" directors and switches near the market. Naturally, with more and more major virtualisation deals now coming up for grabs, each vendor's favourite pastime involves finding flaws in their rivals' offerings. In fact, they're all quite vociferous in thrashing one another's technologies.

Any more objective view however, ought to point out that HDS has won many plaudits for its Tagmastore products as they scale from department through to datacentre level and offer clear migration paths as they scale. It has also been in the market longest.

Then there's EMC. At the high end there's its Invista solution. Among its claims are openness, as it can sit on market-leading Brocade, Cisco, and McData switches. EMC Rainfinity is the company's offering for mid-range to low-end installations. But it has, as yet, no clear scalability path from the low to the high end.

Elsewhere, IBM has its SAN Volume controller, another switch-based product for the enterprise. Its software controls the movement of data through a SAN and offers all the usual virtualisation benefits.

Moving further into the mid-market, appliance-based products tend to grab most of the attention. HP's virtualisation approach is based around partnering with switch OEMs to deliver virtualisation on its StorageWorks storage appliances. Network Appliance is also active in the storage virtualisation appliance.

For its part, Sun Microsystems has a long term partnership with HDS and also offers mainframe level tape virtualisation through its Storagetek range.

For the mid-range its approach is based on partnerships with other OEMs. Tape suppliers including Overland also offer virtualised tape solutions.

At the server software level there are companies such as Datacore, which sells server based software that controls storage assets and manages resources. In short it will control any disk type (SATA, ATA, Fibre channel) across most storage subsystems (EMC, IBM, Sun and more) by sitting on the server and dynamically allocating storage assets to applications.

So what's right for your business?
That depends on your readiness for storage virtualisation, which in turn will depend on the size of your organisation and the scale of its storage infrastructure.

Suffice to say though, that any firm with a sizable, heterogeneous or distributed storage infrastructure should at the very least be examining its storage virtualisation options.

Virtualising even a small part of your storage infrastructure is likely to deliver benefits and fast ROI. More importantly, it will also open the door to the next generation of data management.