Before I go any further I would like to outline my experience and some caveats. Firstly, I’ve been working in the area of thin client computing since the mid-1990s. Before I got into virtualization and VMware, I was a Citrix Certified Instructor (CCI) working initially with Citrix MetaFrame 1.8 on Windows NT4 Terminal Service Edition and more or less ending with Citrix Presentation Server 4.5 on Windows 2003. Before VMware came along and eclipsed my Citrix work – my main product was Citrix. Secondly, I don’t believe in panaceas. There are things I still really love about the Citrix product range, and indeed I still continue to use a Citrix Presentation Server to connect to my remote lab environment, which is held in co-location in the UK. So my message is this – fully research the advantages and disadvantages of ALL the remote desktop and application delivery options now available. When I started if you wanted to deliver a desktop or application to a user down-the-wire there was only ONE real way to do it – Citrix. Now we are bombarded daily with complementary and competing solutions, for example:
- VMware View
- VMware ThinApp
- Citrix XenDesktop
- Citrix XenApp
- Microsoft Remote Desktop Services (RDS)
- Microsoft App-V
- Quest Software vWorkspace
- Xenocode Virtual Application Studio
- Sun Virtual Desktop Connector (VDC)
- HP Client Virtual Software (CVS)
VDI is essentially the same as Terminal Services (TS) or Citrix XenApp (formerly MetaFrame/Presentation server). That is to say, you provide a desktop to the user via a “thin” protocol. The difference between server-based computing and virtual desktops is that rather than having many users connected to one shared TS or Citrix Desktop running a server OS, users connect to their own personal desktop running a desktop OS. By virtue of VDI making use of a non-shared desktop OS, application compatibility issues that can plague a TS environment are almost completely mitigated. A variety of protocols exist to deliver the remote desktop, including the older legacy Microsoft RDP, and the newer VMware PCoIP, Citrix HDX and Microsoft RemoteFX protocols. These new protocols attempt to address some of the persistent graphics rendering limitations of the older display protocols. The advantages of VDI are many, but its key advantages beyond the benefits of Thin Client Computing generally lie in remedying some of the limitations presented by the shared desktop approach of TS and Citrix XenApp.
Advantages of Virtual Desktops
- One user’s activity does not directly affect the performance of other users. Each user is limited to the resources within their VM
- Applications install natively to the Windows environment. There is no need for complicated installation routines and validation to make applications work in an environment for which they were never actually designed. However, many people also like to complement their virtual desktop environment with a virtual application solution like VMware’s ThinApp or Microsoft’s App-V
- Desktop Hardening. The process of locking down the desktop - whilst desirable in VDI - is not mandatory. In TS & Citrix XenApp you absolutely must lock down the desktop to stop one user affecting the stability of the environment for other users using the shared desktop
- VDI allows you to leverage your corporate license agreement with Microsoft at no additional charge depending on if you have a Software Assurance (SA) agreement in place with Microsoft – without it the fee is around $100 per seat., whereas each Citrix XenApp end-user connection requires a license from Citrix. Indeed, Microsoft went so far as to introduce a specific licensing model currently called the VECD (Vista Enterprise Centralized Desktop) program to promote the use of Windows as the operating system in the virtual desktop. This is has been since superseded by a new model called VDA. It’s by no means mandatory that you must use Windows as the guest operating system in a VDI project. You could use a Linux desktop distribution if you prefer it or your needs require it. This said few VDI environments run with just the virtualization layer and a virtual desktop on its own. Nine times out of ten there will be some type of VDI Broker server that will also need licensing!
- VDI can be coupled with other application virtualization tools such as Microsoft’s App-V or VMware’s ThinApp to reduce the footprint of the virtual desktop (because less is installed to Windows) and also allow for advanced features such as being able to run many different versions of the same application (flavours of Microsoft Word and Adobe Acrobat, for instance) on the same virtual desktop
- Features such as VMware View’s Local Mode Desktop that allows an enduser to take a copy of the virtual desktop from the ESX host and make it available on the PC/Laptop even when they are not connected to the corporate network. Local Mode Desktop uses deltas to make sure only changes are synchronized back to the server copy of the VM, and a TTL value that allows for the offline desktop to work only for a limited period. In years to come these Local Mode VMs will be available to a client-based hypervisor as well
- VMware View3 introduced View Composer to enable a linked clone feature. This allows for one single master VM from which many virtual desktops can be created (the linked clone). These linked clones contain only the changes the user makes during the virtual desktop session and, as such, massively reduce the disk space required to run virtual desktops. However, linked clones can introduce other complications around storage capacity management and performance.
Disadvantages of Virtual Desktops
- Printing is a huge challenge in the world of thin-client computing. By far the biggest challenge is the amount of bandwidth used to send a print job from the remote datacentre back to the end-user’s physical printer. It’s quite common to see Microsoft PowerPoint print jobs balloon in size to hundreds of megabytes. Some thin-client vendors have their own solution using some kind of universal PCL printer drivers. Some organizations prefer to buy in a third-party printing solution such as ThinPrint or UniPrint. In View 3, VMware acquired a license for the core ThinPrint product that they call virtual printing. This licensed version of ThinPrint should be good enough to address most printing needs
- The most common VDI protocol is still Microsoft RDP. RDP has been shown not to perform as well as Citrix’s ICA Protocol – and to be especially weak in the realm of multimedia, flash-based web-pages and graphically intensive applications such as CAD. As I mentioned earlier, Microsoft, VMware and Citrix all include new protocols (RemoteFX, PCoIP, HDX) to improve the client protocols used to connect to Windows Vista and Windows 7.
- Storage is quite a significant penalty in VDI. However, with the advent of de-duplication technology from storage vendors such as NetApp, and the introduction of thin provisioned virtual disks in vSphere4, this becomes less significant. As I have already mentioned, VMware had effectively created a kind of built-in de-duplication process with View Composer. If you combine thin provisioning from your storage vendor with thin provisioning from VMware together with the linked clones feature, you are really doing your level best to reduce the disk foot print of your virtual desktop environment. It’s worth stating that for modest VDI solutions local storage is a viable option, but remember that storing a VM on local storage means you cannot use a whole host of VMware features you may take for granted such as HA/DRA/DPM and automation via VUM.
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