Virtual-Server Face-Off

The year 2004 might very well become known as the year of virtualization. Initially, IT pros recognized the value of virtualization technologies in test and demo environments. However, with the advent of ever more powerful systems coupled with continued improvements in virtual machine (VM) technologies, virtualization has since become a production-level technology that enables server consolidation.

VMware was first to market in the virtualization space with the release of VMware GSX Server in 2001. In October 2004, Microsoft entered the virtualization market with Virtual Server 2005, sparking much interest, especially among customers who have come to rely on VM technologies. A comparison of these two titans of virtualization leads to a clear recommendation as to the product that can best address a particular organization's needs.

The VM Architecture
Both products install on top of the base OS and provide a software layer that emulates a physical system. You can install a guest OS on each emulated system, or VM, and you can run multiple VMs concurrently as if each were installed on a separate physical system.

Each VM owns its own virtual hardware, consisting of a processor, disk, memory, and network. VMs aren't aware of other VMs as anything other than networked systems. The virtual server product handles the task of virtualizing the hardware and sharing it with all the VMs. The virtual server also provides virtual networking services that can connect VMs together as well as giving them access to external network resources.

Review Criteria
Although both products possess similar overall functionality, they also have several significant differences. In evaluating the products, the first criterion I considered was the host and guest OSs they support. The host OSs are the OS platforms on which you can install the VM software. The guest OSs are the OSs that the virtual servers can run. I also compared ease of use and overall manageability, looking at the process of creating new VMs as well as the ability to manage the virtual server and the VMs.

Finally, I compared the performance of the VMs running under each product. To check the overall performance of the guest OSs, I used SiSoftware Sandra 2005 Lite benchmarking software (http://www.SiSoftware.net). I compared the results of tests run on the VM that I created on each virtual server—a vanilla installation of Windows Server 2003, Enterprise Edition—to the results of baseline tests I ran on the physical machine. I performed all tests on an HP ProLiant ML350 with dual Intel Xeon 3.2GHz processors, 2GB of RAM, and a dual-channel Ultra320 SCSI controller connected to four 36GB, 15,000rpm hard drives running Windows 2003 as the host OS.

Microsoft Virtual Server 2005
Microsoft makes two versions of Virtual Server 2005: Standard Edition and Enterprise Edition. Standard Edition supports host servers with up to 4 processors; Enterprise Edition supports host servers that have as many as 32 processors. However, the product doesn't provide SMP support for the VMs running on a Virtual Server 2005 server.

By using Physical Address Extension (PAE), Virtual Server can support up to 64GB of memory, and each VM can address up to 3.6GB of memory. Both versions support a maximum of 64 VMs per host. Microsoft supports Virtual Server for use only on 32-bit host platforms.

Unsurprisingly, Virtual Server supports only Microsoft host and guest OSs, as the feature comparison in Table 1 shows. Although I used the product to run other OSs, such as Linux, I don't recommend that you do so in a production environment because of the lack of support.

I'm accustomed to using Microsoft Virtual PC, but Virtual Server is a very different animal and took a little getting used to. Instead of using a Windows-based management console, you manage Virtual Server through the Administration Website console that you see in Figure 1. You access the management program either by selecting the Administration Website option on the Virtual Server 2005 server or by pointing a browser to http://server:1024/VirtualServer/VSWebApp.exe.

Although it took a while for me to feel comfortable with it, the console made it very easy to manage Virtual Server from across the network. The Administration Website provides a thorough overview of the status of the configured VMs, including current performance data and even a mini screen view. I appreciated the no-footprint management offered by the Administration Website, but unfortunately the price you pay for it is having to run IIS 6.0 on the host. The installation process automatically configures IIS, adds the Administration Website, and sets permissions for the site, but you still have an extra element to manage.

Unlike GSX Server, Virtual Server has no wizard to step you through the process of creating VMs. Instead, you need to use the links provided by the Administration Website to manually create a virtual hard disk (VHD), a virtual network, and a VM that utilizes the VHD and virtual network. After I became familiar with the process, I found the interface fairly easy to navigate, but it lacked some of the niceties that I've come to expect from a Windows application, such as the ability to browse the file system when creating VHDs. I like the Administration Website's ability to provide remote control for all the VMs. After you create a VM, you'll probably want to install the Virtual Machine Additions, software drivers that increase screen resolution by adding an SVGA driver and enable better mouse tracking and control.

Virtual Server supports four types of VHDs: dynamically expanding, fixed-size, differencing, and linked. The host OS sees dynamically expanding and fixed-size VHDs as a large .vhd file that contains the file system for the guest VM. Dynamically expanding disks start small and automatically grow as the guest VM requires additional storage. Much like a physical hard drive, a dynamically expanding disk can grow only until it reaches its predefined limit. As you'd expect, the guest VM experiences a delay when the VHD must be expanded. Fixed-size VHDs are allocated when you create them and don't grow.

Dynamically expanding, fixed-size, and differencing VHDs support using an optional undo disk. Undo disks let you reset all changes that have been made to a dynamic, fixed-size, or differencing disk. Undo disks store all configuration and data changes made to the VM during the session and prompt you to save or discard the changes when you shut down the VM. Differencing disks let you isolate changes that occur within a guest VHD; all changes that occur in the parent VHD are stored in the differencing disk. Unlike an undo disk, which is associated with the entire VM, a differencing disk is associated with a particular VHD. Compared with GSX Server's differencing disks, Virtual Server's built-in differencing disks are a snap to create.

Linked VHDs are different from the other types of VHDs. Linked disks convert an entire host file system's partition to a VHD. Afterward, the host can no longer access that portion of the file system. You can't use linked disks with undo disks or differencing disks.

You can configure virtual networking to use either the host system's NIC or a user-defined virtual network that only VMs can access. If you use the host NIC, any VM connected to the virtual network can access the network that the host is connected to. Otherwise, the VM can access only the internal virtual network. Virtual Server can also provide a virtual DHCP server, so you don't need to configure a guest VM on an internal network to act as a DCHP server.

One especially nice feature is Virtual Server's ability to configure shared SCSI VHDs, which lets you set up Microsoft Cluster service over two VM nodes. Another welcome feature is the ability to transfer VMs created with Virtual PC 2004 to Virtual Server. One annoying limitation of Virtual Server is that, like Virtual PC, it lacks support for USB devices. Although you can use USB keyboards and mouse devices, you can't plug in USB flash drives with Virtual Server and have them recognized in the VMs. Virtual Server also has a strong set of COM-based APIs that you can use in conjunction with VBScript to create your own custom management scripts.

Microsoft offers the Virtual Server Migration Toolkit (VSMT) as a free add-on to Virtual Server. Available for download at http://www.microsoft.com/widowsserversystem/virtualserver/evaluation/vsmt.mspx, the VSMT can convert physical machines to VMs and VMware VMs to Virtual Server–compatible VMs. VMware offers a similar product, called the VMware P2V Assistant, but you must purchase it separately.

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