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How server virtualization makes storage management more difficult, and what IT departments can do about it

Pick any business or government organisation at random and there’s a good chance that they are saving money by using server virtualization. However, as is now becoming clear, there is also a chance that they are blowing those savings on extra storage capacity as a result.

In a January 2010 report from US-based investment house William Blair & Company, analysts Jason Ader and Dmitry Netis estimated that, for every $1 spent on server virtualization, customers spend between $2 and $3 on storage. No wonder, then, that they described server virtualization as “the key technology development that will shape the future of the storage industry”. 

That is good news for storage vendors, but bad news for cost-conscious customers. In some cases, the incremental spend on storage is understandable: in order to unlock the full potential of server virtualization technology, and in particular the ability to move virtual machines (VMs) between physical hosts, networked storage is required.

For companies just starting out on virtualization, that means implementing a storage area network (SAN). For many others, it demands an upgrade of the existing SAN environment. But that is not the end of the story. Even those organisations that have invested significantly in their storage infrastructures in recent years are running into problems with server virtualization.   

Trouble ahead

First, they are encountering big issues with storage utilisation, according to Adam Stringer, an expert in storage systems at management consultancy PA Consulting. “There’s a great deal of wasted capacity in many virtualized server estates,” he says.  

In part, that is because virtualization allows VMs – and the disk storage they require – to be deployed rapidly, leading to the situation commonly referred to as ‘virtual sprawl’.  “A VM that is no longer needed but remains on a host system in the server estate has storage allocated to it that could otherwise be deployed elsewhere,” he points out.

But the situation is further exacerbated by the fact that most VMs are created from a template, a so-called ‘gold image’, in order to speed up deployment. When creating these templates, most IT teams set the default size for the virtual machine disk image (VMDK) at a relatively high number – on average, between 50 and 100 gigabytes – in order to accommodate the operating system, application and data storage required.

The problem here is that the space required by different VMs in fact varies greatly. By setting the default VMDK high enough for an organisation’s more disk-hungry applications, systems administrators may well be lavishly over-provisioning others.

According to Keith Inight, technology strategy director for global managed operations at IT services company Atos Origin, this over-provisioning is often seen as “the better of two evils, since most applications will crash ungracefully once they run out of available disk space”.

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