Virtual desktop infrastructure (VDI) has so far failed to take off in a big way, and one reason could be because of the storage challenges it raises -- almost half of all VDI pilot projects stall because of storage cost and performance issues. That's according to Virsto, a California-based company that specializes in storage virtualization software.
This week at Citrix's annual Synergy bash, the company unveiled its "Virsto for XenDesktop on vSphere" storage software. It promises that for XenDesktop VDI implementations the software, which Virsto confusingly calls a storage hypervisor, can improve storage throughput by a factor of ten, lower the storage costs per desktop by more than 50 percent, and double the density of XenDesktop virtual desktops you can pack on to a single physical host.
While it certainly sounds good, so far Virsto has only released a private beta of the product, so there is no way of independently verifying these claims. Virsto does have existing similar products for server virtualization implementations using Microsoft's and VMware's virtualization technology platforms, as well as products for VDI implementations on the Hyper-V hypervisor, so it's certainly worth taking a look at just what it is that Virsto is doing with its so-called storage hypervisor software.
Virsto's software has to be installed on each physical virtualization host on top of the vSphere hypervisor, where the software appears as a virtual appliance. It's about 10GB in size, so it takes up a small but certainly non-negligible amount of the host machine's resources.
Gregg Holzrichter, Virsto's VP of Marketing, explains what the software does. "Once it's installed, it takes the I/O from each VM on the host. So instead of it going through the hypervisor, it intercepts it and handles it more efficiently, removing storage bottlenecks. We get around the native way VMware deals with I/O."
He says that Virsto's approach is simply continuing the trend of adapting storage technologies to virtualization: from storage arrays from the likes of EMC and NetAPP, to SSD appliances close to the server from newer vendors such as Tintri, Nimble, Nutanix and StorSimple, and technology right inside the server from the likes of Fusion I/O, EMC VF Cache and STEC.
"We go even closer, presenting storage at the VM level," says Holzrichter. The actual storage capacity is located on external block-level storage and is "checked in" to contribute capacity to a pool that Virsto calls vSpace.
Holzrichter points out that in normal circumstances storage admins allocate up to 50GB of storage capacity per VM to ensure there's no contention -- and therefore high performance -- even though each VM actually uses a small fraction of that space. That means there's a huge amount of unused capacity in any organization with a large number of VMs.
Here's how he says Virsto gets around this. Once Virsto's software has been installed on vSphere, it appears in a tab in vCenter. Storage administrators can then right click on a VM and point it at Virsto. Virsto creates a virtual storage layer that presents virtual hard disks to virtual machines.
Virsto vDisks look like native Eager Zeroed Thick (EZT) disks, but consume only a fraction of their physical storage space. "The VM thinks it is being provisioned, but we are only writing blocks that are being written through. Ultimately this saves a huge amount of storage -- you get perhaps ten times more out of it than before."
The company claims that in a real-world VDI implementation, one customer achieved a 70 percent storage performance increase and an 80 percent reduction in storage requirements using the software. "The problem with VDI is that the challenge of writing to storage is much more pronounced than with many server virtualization applications," says Holzrichter. "You can often get 25-50 desktops on a physical host, but with Virsto we can double that. "
Virsto for XenDesktop on vSphere delivers native support for vSphere 4.1 and 5.0, and will be available in Q3. Pricing is based on the amount of storage capacity checked in to a vSpace -- 10 terabytes costs $50,000, plus a 20% annual maintenance fee.
The big question that remains to be seen is whether improvements in the economics and performance of storage for virtualized environments such as those offered by Virsto can give VDI the boost it needs to move from a technology that's stuck in the proof of concept stage to one that becomes widely adopted.
Paul Rubens is a technology journalist and contributor to ServerWatch, EnterpriseNetworkingPlanet and EnterpriseMobileToday. He has also covered technology for international newspapers and magazines including The Economist and The Financial Times since 1991.