Article

With server virtualisation, storage often suffers

Bryan Betts, Contributor

The growth of virtualised server environments seems unstoppable. According to a recent survey by IDC, the proportion of organisations using server virtualisation grew from 46% to 54% this year, while at the same time the use of server virtualisation technology also grew within those organisations.

"Virtualisation use has exploded since our last survey of the European market," says Chris Ingle, IDC consulting and research director with IDC's Systems Group in London. He adds that 35% of the servers purchased in 2007 were virtualised, but this year that number is predicted to rise to 52%. Ingle says that more than half (54%) of the organisations not yet using server virtualisation expect to do so in the next 18 months.

Adding a virtualisation layer (or hypervisor) to a server allows several lightly-loaded servers to be consolidated into one box by converting each server into a virtual machine (VM). But along with straightforward consolidation, Ingle says organisations are using server virtualisation to improve disaster recovery and application availability. That's because a VM could also be moved to a more powerful server, backed up whole, or mirrored to a second site.

"We have demonstrated to our clinical staff moving a server in real time with no problems," confirms Dr Zafar Chaudry, CIO at Liverpool Women's NHS Foundation Trust. "It takes us just 10 to 15 minutes to commission a [virtual] server, too."

We can tell when people acquire server virtualisation products, because calls to our support line go up.

 

Praveen Asthana
Enterpirse Storage DirectorDell

 However, when you move a VM to another server, its SAN and LAN access has to be reconfigured. If you stay within one virtual server framework -- VMware, in Chaudry's case -- and accept the management and storage overheads that come with that, then moving a virtual machine is simple enough.

But maybe you want to automate the process of moving VMs for server load balancing, as proposed by companies such as HP and Scalent V/OE. Or perhaps you want to use a second hypervisor in parallel, such as Microsoft Hyper-V, Virtual Iron, Parallels Virtuozzo Containers or Xen.

Or you simply want to do more with your storage. For example, VMware suggests that its users should allocate twice the amount of disk space that they actually plan to write to, but SAN virtualisation tools could let you use thin provisioning to avoid that additional hardware expense.

Now it's getting complex -- and virtualising your servers isn't plain sailing any more.

It adds risks too. Security is an obvious one, as you must ensure that VMs cannot see each others' data. You will need to keep full image copies of VMs too, so they can be reconstituted on a new server if the first one fails.

Then there is the inevitable performance hit: The VMs have to share their server's physical interfaces, and the hypervisor must manage that sharing process and act as a virtual network switch.

"A lot of our customers have jumped on the virtualisation bandwagon and then had an 'Oh shoot!' moment when their storage doesn't work," says Praveen Asthana, Dell's enterprise storage director. "We can tell when people acquire server virtualisation products, because calls to our support line go up."

Asthana argues that the problem is more with Fibre Channel SANs than iSCSI: "It's the way Fibre Channel tends to work with virtual servers, it is physically-protected and pre-dates virtualisation. The hypervisor therefore has to work as an arbitrator, mapping between the VMs and LUNs.

"But iSCSI can bypass the hypervisor because it's virtually-protected, so there is no mapping issue," Asthana says. "For example, you can connect directly to the Microsoft iSCSI initiator on a VM. We found that iSCSI users doing virtualisation were not calling us."

There can still be issues of visibility and performance though, says Marcel Dumont, the operations director for Dutch social networking site Schoolbank, which uses iSCSI storage. He says the site's developers test on virtual machines, and overall system performance can suffer badly if one VM does a lot of small random I/Os. However, he can't view the I/O loading of an individual VM, so he has to use indirect methods, watching the processor loading instead and moving the VM to a different server if necessary.

Requiring the hypervisor to act as a virtual network has limited users' ability to virtualise network-intensive applications. Fortunately, Vvirtualising Fibre Channel HBAs and Ethernet adapters are providing ways to overcome this and the security issues.

These devices are produced by the likes of Chelsio Communications, Neterion and NetXen on the Ethernet side, and by companies such as Emulex and QLogic for Fibre Channel. They not only offload the host machine by processing the network protocols and packets in hardware, but they emulate multiple adapters, giving each VM - and a modern multi-core server can host dozens of VMs - its own virtual network connection.

"Virtualised connections also make it easier to create VMs or move them to other servers, so they're pretty much essential if you want to automate your data centre," says Juergen Arnold, the chairman of storage industry body SNIA-Europe. "At the end of the day," he says, "it's not virtualisation that brings the big savings, it's automation. And virtualisation is just an enabler to that."

About the author:Bryan Betts is a journalist based in West London who specialises in business and technology. He was one of the first freelance writers in the UK to specialise in storage technology, from SANs right down to the level of disk heads, platters and motors.


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