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Virtual desktops and storage

Server virtualization has had a profound impact on storage infrastructures, but the coming wave of desktop virtualization will place new demands on storage environments.

Server virtualization has had a profound impact on storage infrastructures, but the coming wave of desktop virtualization will place new demands on storage environments.

Desktop and laptop computers are among the most difficult IT assets to manage. Because they reside with users, and are numerous and distributed by nature, many companies simply defer critical desktop management tasks, such as data protection and patching, to users. That may seem like a practical strategy to some organizations, but it poses great risks in the areas of compliance, security and IT governance in general. Products to address these issues are plentiful -- from expensive desktop management applications to client-side backup and security tools -- but they're mainly point solutions and can be costly.

For storage managers, virtualizing desktops should be a real concern as all of the operating systems, applications and data currently residing on desktop and laptop PCs will need to find a new home in the data center storage environment. The required storage resources, as well as their ongoing management and administration, could be staggering.

VDI promise and challenges

Virtual desktop infrastructure (VDI) -- Stamford, Conn.-based Gartner Inc. categorizes it as hosted virtual desktops -- has the potential to cure this ongoing IT ailment by moving desktops into the data center. Instead of booting a local client-side operating system (OS), users connect via a browser or thin client to a virtual desktop that runs as a virtual machine (VM) on a server in the data center. Consequently, ownership of the desktop transfers from the user to IT and, by centralizing desktops, the long list of daunting desktop management tasks is greatly reduced.

Wide adoption of VDI beyond a limited number of use cases has been stifled thus far by its lack of mobility and the need to stay connected. But that's about to change. Citrix Systems Inc. and VMware Inc. have been developing offline VDI technologies that enable users to take virtualized desktops on the road while remaining secure and compliant with centrally managed policies. Offline capabilities have the potential to catapult virtual desktop infrastructure from a niche application to mainstream deployment. While Citrix Systems has partnered with Intel Corp. to develop the bare-metal Citrix Thin Client client-side hypervisor, which runs the virtual desktop on the client while disconnected from the server, VMware has released an experimental version of its Offline Desktop software, which enables users to check out their desktop and take it on the road.

"For VDI to become widely adopted in the enterprise, mobility and the ability to take desktops offline are instrumental, and we should see offline VDI production deployments by 2010," said Mark Bowker, an analyst at Milford, Mass.-based Enterprise Strategy Group (ESG).

A second key requirement for VDI to prevail is user experience. Virtual desktops need to have the look-and-feel and flexibility of traditional thick clients. Unless audio, video and graphics capabilities, and peripheral support is comparable to (or at least close to) traditional desktops and laptops, virtual desktop infrastructure is likely to continue its niche existence and share the fate of the very thin, but relatively static, terminal server-based computing approach. But great efforts have been put into the Remote Desktop Protocol (RDP) to close the user experience gap between thick clients and VDI clients. Citrix Systems, with its Independent Computing Architecture (ICA) protocol and High Definition Experience (HDX) extensions, currently has the most comprehensive multimedia and peripheral support. Comparably, VMware has partnered with Teradici Corp. to develop a software-only version of PC-over-IP to deliver multimedia to VDI clients beyond the rudimentary capabilities of the standard RDP protocol.

"Today VDI is ready for some users, but it'll probably take another 18 to 24 months before most of the challenges related to protocol limitations, performance and offline support get resolved," said Brian Gammage, a vice president/Gartner fellow who covers client computing.

VDI vendors and products



Citrix Systems Inc.

Microsoft Corp.


VMware Inc.




Virtuozzo Containers

VMware ESX

Virtual desktop infrastructure (VDI) product


Licensed from Citrix Systems

Depends on third-party session brokers: Ericom Software's PowerTerm WebConnect and Quest Software Inc.'s vWorkspace

VMware View

Offline capabilities

Via Citrix Thin Client by late 2009 or 2010

Will leverage Citrix Thin Client when available


Via VMware Offline Desktop (available as experimental release)

Application virtualization



Via application templates


VM image sharing

Via Provisioning Server

Licensed from Citrix

Built into Virtuozzo Containers

Via VMware View Composer linked clone feature

Biggest benefit

Most advanced desktop and application virtualization products

Hyper-V is part of Windows Server 2008

Lowest infrastructure requirements; cost-efficient and low degree of complexity

Strongest and most widely deployed hypervisor


Storage resources needed for VDI

The final and probably biggest challenge for VDI to succeed is its infrastructure impact and cost. With desktop and laptop prices starting at $500, plus intangible management costs, any ROI for virtual desktop infrastructure needs to weigh the obvious benefits against the cost and effort it takes to acquire and maintain a data center infrastructure that can support hundreds and possibly thousands of virtual desktops.

To start, VDI requires servers to run the desktop virtual machines. With one CPU core able to support about five users, according to Gammage, a typical eight-core server can support approximately 40 users. VDI requires network storage for desktop virtual machine images and user data. By simply moving desktop images from clients to servers, and with the average desktop image requiring approximately 10 GB of storage, 100 virtual desktop images consume 1 TB of relatively expensive network storage vs. the low-cost disks used in contemporary desktops and laptops.

Even with offline support, VDI depends on a reliable network and sufficient bandwidth on both the local-area network (LAN) and wide-area network (WAN), and the larger the VDI deployment the more likely network upgrades will be required. Add to that the cost of cooling, electricity and data center space, and it adds up to a significant IT infrastructure investment. On top of that infrastructure investment, there's the cost of the virtual desktop software, typically licensed by concurrent desktop (VMware View) or concurrent user (Citrix XenDesktop), OS licenses, as well as the Microsoft Corp. Vista Enterprise Centralized Desktops (VECD) add-on license Microsoft demands for each VDI client in addition to the OS license.

The cost of getting a virtual desktop infrastructure off the ground is truly staggering, and VDI software vendors have been working feverishly to minimize hardware requirements, especially those related to storage. They're also partnering with server, storage and networking vendors to maximize the number of virtualized desktops their devices can support by adding features like data deduplication and thin provisioning to storage arrays, and enhancing servers to increase the number of concurrent VDI sessions the servers can support. One example is Cisco Systems Inc.'s Unified Computing System (UCS) server platform.

"While a normal server blade can support about 40 to 60 VDI clients, UCS more than doubles that number, effectively reducing the cost per user by more than a factor of two," noted Chad Sakac, vice president of the VMware Technology Alliance.

To VDI or not to VDI

1. Any virtual desktop infrastructure (VDI) deployment consideration needs to start with a thorough return on investment (ROI) analysis that weighs desktop management and compliance benefits against the cost of deploying and maintaining a VDI infrastructure.

2. Because IT is more likely considered a necessity rather than an asset at small- and medium-sized businesses (SMBs), VDI may not be an option for these types of firms with their limited IT teams and modest data centers. As VDI service providers mature, outsourced virtual desktop infrastructure and VDI in the cloud will become viable options for smaller firms.

3. When evaluating a VDI platform, focus on advanced features, such as offline support, linked clone-type capabilities for efficient use of storage and simplified management, as well as application virtualization capabilities. These are the areas where you get the biggest bang, but it's also where products differ the most.

4. Keep your mind open to mixing hypervisors and VDI products. While there's a small benefit to having all components from a single vendor, mixing and matching different vendors to get best-of-breed products or the most cost-efficient combinations is a valid and prudent option.

5. Start with a small number of desktops and grow your VDI footprint from there. Because it's such a radical change, a gradual rollout that includes frequently reassessing the project and making adjustments will increase your likelihood of success.


VDI architecture and platforms

VDI comprises several joined components that enable clients to run their desktops remotely from within the data center. One of the key elements is a hypervisor, such as Citrix XenServer, Microsoft Hyper-V or VMware ESX Server, which runs on a server and executes the desktop virtual machines. A second ingredient is a management platform offered by the hypervisor vendors for their respective products. This manages the hypervisor servers as well as the pool of VMs used by existing connections and requested by new connections, and facilitates provisioning desktop virtual machines to VDI clients. The third element is a session broker, which is responsible for distributing sessions from clients to VMs and redirecting users of disconnected sessions back to their original virtual machines. For a VDI client to start up, these three components work hand-in-hand: On receiving a request for a new session and after successful authentication -- typically via Active Directory -- the session broker asks the management platform to start up a virtual machine -- provisioned with the OS, applications and profiles assigned to the connecting user -- that establishes a remote desktop connection between the client and desktop VM on the hypervisor server. While all VDI platforms are architected in this fashion, vendor implementations vary significantly, each with its pros and cons.

Citrix XenDesktop's Provisioning Server acts as a central proxy for all virtual desktops and is used by admins to build, configure and manage desktop virtual machine images. The XenDesktop Delivery Controller (DDC) is the session broker and manages user access to desktop virtual machines. Besides being one of the most advanced VDI platforms, XenDesktop greatly benefits from the popularity and strength of Citrix XenApp (Citrix's terminal server-based platform), which allows applications to be streamed for local and offline use, and is often used to complement XenDesktop deployments.

VMware View is on par with Citrix XenDesktop in most areas, but lags in user experience, at least until VMware releases its version of PC-over-IP to match or exceed Citrix's ICA protocol and HDX extensions. VMware View includes VMware ThinApp for application virtualization, but unlike Citrix XenApp's flexible concurrent online and offline application support, its application delivery functionality is focused mostly on isolating applications from the operating system and other applications.

Microsoft has so far played a role in virtual desktop infrastructure with its Hyper-V hypervisor and System Center Virtual Machine Manager (SCVMM) management platform, but has relied on its partnership with Citrix Systems for a session broker. Microsoft will include a session broker in Windows 7, but it's intended for smaller deployments. On the app virtualization front, Microsoft is offering Application Virtualization (App-V), formerly known as SoftGrid. Like VMware ThinApp, it's more limited than Citrix XenApp and focused primarily on isolating apps from each other and the OS.

Leadership in the VDI market is currently shared by VMware and Citrix Systems. While VMware boasts the widest deployed hypervisor platform, Citrix Systems, with a 90+% market share, according to Gartner's Gammage, dominates server-based desktop and app computing. Luckily, and mostly the result of an interoperability agreement Microsoft and Citrix Systems signed in 2006, customers can mix and match virtual desktop platforms and hypervisors. "Many of our customers run XenDesktop with Microsoft or VMware hypervisors," confirmed Calvin Hsu, director of XenDesktop product marketing at Citrix Systems.

Parallels has a VDI product that's based on its Parallels Virtuozzo Containers virtualization offering. Contrary to competing hypervisors that require a full OS image per VM, virtual containers -- the equivalent of virtual machines -- leverage a single OS. Instead of having resources assigned to each container, all containers share the resources of the host operating system. On the downside, Parallels VDI product doesn't have the clear-cut OS isolation, and is less flexible and more proprietary than traditional hypervisors. Similar to Microsoft, Parallels doesn't have its own session broker and relies on third-party brokers such as Quest Software Inc.'s vWorkspace and Ericom Software's PowerTerm WebConnect.

The VDI storage challenge

Unlike traditional hypervisors, Parallels' Virtuozzo Containers virtualization product is architected to minimize storage requirements. Any newly created virtual container references the underlying OS, and it's only as the OS and applications within each virtual container change that changes are stored within each container's virtual disk. "We can create a 50 Gig virtual disk, but the actually used space may only be a few megabytes and [it] grows as files change," explained Jamie Moore, senior sales engineer in the enterprise group at Parallels. Because of its efficient use of storage, Virtuozzo works well with both networked and direct-attached storage (DAS), making it very attractive for smaller environments.

Both Citrix Systems and VMware realized that for their VDI offerings to succeed they had to overcome the detriment of having to store and manage full VM images for each virtualized desktop, and they did. But unlike Parallels, they implemented their solutions outside the hypervisor.

Citrix Systems has solved the problem within its Provisioning Server. It supports creating one or several golden virtual machine images, as well as VM templates that define resource configurations such as memory, disk space and I/O devices. Once a golden VM image is created, it can be used for multiple virtual desktop users. Virtual desktop changes that occur during a user session are stored and retained in a so-called write-cache. "You can get hundreds of virtual desktops running off the same image, with literally 100% savings on disk space," noted Citrix Systems' Hsu. Different from Parallels, the write-cache feature doesn't work with DAS but requires shared storage.

VMware has addressed the storage issue with its VMware View Composer, a new component in VMware View 3. View Composer uses VMware linked clone technology to create desktop images that share virtual disks with a master image to conserve disk space. Individual desktop VM images are linked to a master virtual machine image and, as a result, can simply be patched or updated by updating the shared master image, all without affecting user-specific settings, data or apps. During runtime, VMware View Composer combines the master image with changes the user made. "View Composer reduces storage needs and costs by up to 70% while simplifying desktop management," VMware's Sakac said.

Storage vendors focus on VDI

Besides the aforementioned virtual desktop infrastructure capabilities, VDI users are leveraging advanced array features, such as data deduplication and thin provisioning, to reduce the disk space required for VM images and user data. Because many NAS arrays support data dedupe, and desktop VM images and user data are file based, network-attached storage is a natural fit for VDI. A case in point is offered by Dave DePillis, manager of IT operations at Allied Cash Advance, a Miami consumer loan firm. He runs all of his clients on VMware View 3 and stores desktop images and user data on NetApp FAS2020 arrays. "We're not using linked clones at this point because of some limitations, but instead rely on NetApp's deduplication to preserve disk space. After deduplication, we gain back about 85% of the allocated storage," DePillis said.

As VDI capabilities like VMware's linked clone and Citrix Systems' equivalent feature in Provisioning Server mature, they'll be the preferred method of reducing disk space for virtual images because they provide the added benefit of having fewer desktop images to manage. Rather than managing one desktop image per virtual desktop, desktop management is reduced to managing one or a few master images. Patches, upgrades and configuration changes applied to the master image will be automatically propagated to virtual desktops that are linked to the master image. The impact on desktop management is huge and only compounded as the number of virtual desktops grows. "I hope to eventually be able to leverage VMware's linked clones to simplify patching and software upgrades; at that point the NetApp deduplication will mostly benefit user data," Allied Cash Advance's DePillis said.

VDI poses something of a storage paradox. It calls for relatively low-end storage to make the ROI work, but it also needs to be highly available. "Our customers say that they want tier 1 availability at tier 6 pricing for VDI storage," VMware's Sakac quipped. As a result, VDI deployments are likely to shy away from expensive Fibre Channel SANs and gravitate toward less expensive iSCSI SANs, a notion confirmed by Hewlett-Packard (HP) Co.'s acquisition of iSCSI pioneer LeftHand Networks Inc. in 2008. "Its built-in replication, support of thin provisioning and the simplicity of iSCSI make the HP LeftHand P4000 SAN a perfect fit for VDI," explained Lee Johns, director of marketing for HP's StorageWorks Division.

The larger the number of virtual desktop users, the higher the risk of network or storage contention. Most feared is the so-called boot-storm that occurs when a large number of users fire up their virtual desktops. With features like linked clones that cause many users to access the same virtual machine image on the same blocks and spindle, a serious performance bottleneck is inevitable unless the storage array has plenty of cache. So any storage array used for desktop VM images should have an option to add cache if needed. "All NetApp filers with PCI slots have the option of adding one or multiple Performance Acceleration Modules [PAMs]," explained Manish Goel, NetApp's senior vice president, product operations. At this time, each PAM can add 16 GB of read cache for a total of 80 GB. NetApp said future expansion will allow a maximum cache of 512 GB.

Gathering momentum

From maturing VDI products, impending offline capabilities, Microsoft's addition of a VDI session broker in Windows 7 and Intel's chip-side support for client-side hypervisors, to emerging VDI service providers that offer VDI as a managed service, the momentum behind VDI is gaining strength. Because of its infrastructure requirements and because it represents a radical change to managing desktops, wide adoption is all but certain. The stars are lining up behind VDI to cause a major change in how companies will manage desktops in the future.


BIO: Jacob Gsoedl is a freelance writer and a corporate director for business systems. He can be reached at

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