The problem with today's most powerful computer hardware is that it was designed to run a single operating system to support a single application. This approach leaves most of the machine resources underutilized. Server consolidation through virtualization gives us the opportunity to utilize and manage these resources efficiently to move towards a more cost-effective and manageable environment.
This report provides real world information about concepts and features of virtualization technologies with the objective to help us derive the best candidate for a consolidation project. The research was conducted on XenServer 5, VMware ESXi 4, and Microsoft Hyper-V technologies to compare product features, accessibility and manageability. The information was derived from reliable resources found on the Internet and from related books.
2. Virtualization Overview
Virtualization is a broad term which is applied to many different applications in today's computing. According to Springboard Research, a growth of $1.35 billion is expected for 2010 (Nagel, 2008). Virtualization hides the physical characteristics of computing resources from the way other systems, applications and users interact with such resources. Virtual memory is an example of virtualization and is a common part of most operating systems.
The hardware of computer systems is based on the x86 instruction set and was designed to allow a single operating system to directly access the hardware resources, so virtualization was not supported (Smith & Navi, 2004). However researchers at the Stanford University came up with the idea to replicate the x86 ISA architecture using software emulation technologies. These researchers founded a company named VMware, which is now one of the virtualization market leaders (VMware, 2009a).
2.1 Types of Virtualization Hypervisors
The Hypervisor is a software layer that runs directly on bare hardware or as part of the OS to allows concurrent execution of multiple, different operating systems on the same hardware platform. There are two major types of Hypervisors, Type 1 and Type 2. The Type 1 Hypervisor, also known as Native or Bare-Metal Hypervisor is installed directly on top of the system's hardware platform. Virtual Machines are then created on top of the Hypervisor to host the guest operating systems (Vanover, 2009). The architecture of Type 2 Hypervisors is different as it runs inside a pre-installed operating system as illustrated Figure 2.1.1. This method adds an extra layer to the virtual structure and guest operating systems run on a third level above the hardware (Vanover, 2009).
Physical Server
Hypervisor /VMM
Hardware
VM
VM
VM
APP
APP
APP
Guest OS
Guest OS
Guest OS
Physical Server
Hypervisor /VMM
Hardware
Guest OS
Guest OS
Guest OS
VM
VM
VM
APP
APP
APP
Hosting OS
Type 1 - Bare Metal Hypervisor Type 2 - Hosted Hypervisor
Figure 2.1.1 A simplified view of the Type 1 and Type 2 Hypervisor architecture.
2.2 Bare Metal Hypervisors
VMware Hypervisor was the market share leader with 89% , Microsoft Hyper-V with 7% while the XenServer from Citrix had only 2% in 2009 (VMware News Releases, 2009) (Server Virtualization News, 2009). However, the report by ITIC shows that Microsoft will be the favorite virtualization application, for the coming future (Johnston, 2009).
2.3 VMware ESXi Server
The ESX Server has its own proprietary operating system that installs directly on the hardware. It was designed to give the hosted VMs the best performance possible with full control on how resources are shared and utilized. The ESXi Server is made up of two core components, the VMkernel and the VMnix. The purpose of the VMnix is solely to interact with the server via SSH or HTTPs while the VMkernel manages and schedules access to the hardware platform for the hosted VMs as shown in figure 2.3.1 (VMware Inc, 2009).
ADMIN
ESX Server Architecture
VMkernel
Host Hardware
Processor Memory Disk Network
Console
Operating
System
Virtual Machines
Admin Access
SSH or HTTPs
Figure 2.3.1 A simplified view of the ESXi Server architecture.
The ESX kernel is what makes it different from all the other Hypervisor software packages. It is extremely light with an estimated overhead of approximately 3 to 8 percent (Oglesby, Herold, & Mike, 2007).
2.4 Microsoft Hyper-V Server
Microsoft released Hyper-V as their first Bare-Metal Hypervisor virtualization technology on March 2008 with a newly developed microkernel. Hyper-V is made available as an integral part of the Windows Server 2008, with less than 1MB in size and is only supported on x64 bit platforms. Most of the tasks are delegated to a Parent Partition which automatically loads a full copy of the new Windows Server 2008 core as shown in Figure 2.4.1 (Adhikari, 2008). Hyper-V provides isolation between guest operating systems and allows the consolidation of multiple server roles as separate VMs running on a single platform. It is very scalable and is calculated to increases the disk input/output activity by 107% (Perilli, Microsoft Hyper-V, 2008).
HARDWARE
HYPERVISOR
VM Bus
Parent
Partition
Parent
Partition
Parent
Partition
Parent
Partition
Drivers
Figure 2.4.1 Simplified view of the Microsoft Hyper-V architecture.
2.5 XenServer Hypervisor
The XenServer Hypervisor sits directly on the hardware platform, below any operating systems and is responsible for CPU scheduling and memory partitioning for the various hosted VMs. The network, storage, and the other I/O functions found on a hardware platform are controlled by the Domain 0 drivers, which is an essential VM that runs on the XenServer Hypervisor as shown in Figure 2.5.1 (Abels, Dhawan, & Chandrasekaran, 2005). Two drivers are included in Domain 0 to support network and local disk requests from Domain U PV and HVM Guests (Abels, Dhawan, & Chandrasekaran, 2005)
Hardware
XEN Hypervisor
Domain 0
Dom U
PV GUEST
Dom U
HVM GUEST
XEN DMC & C
Network Driver
Block Driver
Figure 2.5.1 Basic architecture overview of the XenServer Hypervisor.
3. Critical Analysis
The analysis were carried out to compare and evaluate the Hypervisor products against accessibility, features, functionality and manageability based on previous analysis and tests performed by reliable resources derived from the Internet.
3.1 Product Accessibility
Each product was assessed based on how easy it is to obtain, install and on support provided by the product vendors and from other sources.
Citrix XenServer
The XenSever is easily downloaded from the Citrix product Web Page after filling and submitting some personal information details. A single Linux ISO install file is required to install the server. The XenCenter management utility is also installed from the same installation CD (Citrix, 2009).
The installation of the XenServer is very similar to the Red Hat Linux and takes approximately 10 minutes. The server is simply booted from the installation CD and details such as the hostname and the IP address are entered (XenSource, 2009). Product support is available from the XenSource community and via forums (Citrix, XenServer Essentials, 2009).
VMware ESXi 4
VMware ESXi 4 ISO image is also downloadable from the VMware Web Site. The installation is very straightforward. The system is booted from the installation CD and the user is presented with a most polished GUI, reflecting this product's maturity. The management console's interface is Web based and must be downloaded separately (VMware, 2010).
The overall process takes a little bit longer than that of the XenServer installation. It has the most advanced features when the full license is purchased with a strong roadmap and the best upgrade path. (Finnis, 2009).
Very good support is provided from certified engineers and via forums, as well as many advanced tools are available to assist in the migration process from the physical to the virtual environment (VMware, 2010).
Microsoft Hyper-V server
Unlike the ESX and the XenServer, Hyper-V is wrapped within Windows Server 2008. The download and installation are pretty straight forward (Microsoft, 2010). The configuration of the Hyper-V is done via the terminal screen which options are quite tedious to understand especially for the novice user. An added MMC snap-in must be installed separately on a Vista or on a Windows server machine as Windows XP is not supported (Microsoft C. , 2010).
The implementation time is very difficult to predict, updates need to be immediately applied in order to get it up running with some other tune ups, such as firewall and services configurations (VirtualPro, 2008). When it comes to product support, Microsoft training and development curve translates into a large number of trained and certified individuals, so support should not be a problem in the future (Microsoft, Virtualization with Hyper-V, 2009).
3.2 Product features and functions
Citrix XenServer
XenServer has the best capabilities and features of the three products, however it does not have features like memory-over-commit and consolidated backup. Similar to VMware ESX server, it is installed and managed like an appliance rather than like an OS. It is supported by a wider range of hardware platforms and is unique in supporting up to eight vCPUs (ITReviews, 2009).
The free edition provides much more features that VMware and Hyper-V. Windows guests are well supported, but the supported Linux guests list is tighter when compared to VMware, Ubuntu is not supported. According to tests performed by Enex Labs non- supported Linux guests don't seem to run well at all (Jones, 2009).
VMware ESXi 4
VMware ESXi server differs from XenServer and Hyper-V in that it does not require Pacifica or VT extensions, as it uses pure software based virtualization (ITReviews, 2009). The server has the biggest list of supported guest operating systems with one unique feature, memory-over-commit, which allows more virtual memory allocation to VMs than there is physical memory.
However, ESXi has a tighter list of compatible hardware and requires a networked clustered file system to store the VM images known as VMFS. VMware provides a free version of the ESXi server with limited features, which can adopted for standalone environment (Jones, 2009).
Microsoft Hyper-V Server 2008
Hyper-V can run on practically any 64-bit hardware that supports Windows based drivers and fits very well in windows domain infrastructures. The VMs are stored on a regular NTFS partitions and provides the Parent OS as a pass-thru mechanism for built-in driver support. (Perumal, 2008).
Unlike the ESX server, the free version of Hyper-V comes with a good feature set including clustering and live migration. Hyper-V runs Windows guest operating systems very well and even Linux guests seem to run reasonably well. However, Hyper-V is very difficult to manage in a non domain environment (Jones, 2009).
3.3 Product Manageability
Each product was assessed against the ease of management of the host server, together with the tools provided by native system console to facilitate the administration of the hosted virtual machines.
Citrix XenServer
The XenServer is managed by the XenCenter management suite which provides the most features of the three products. However the management console is a Windows-based application as opposed to the other web-based consoles. The console can be used to create, backup, copy entire VMs, migrating of VMs and for creating resource pools to set up high availability. Configurations and performance statistics are built in and are easy to configure (Vugt, 2010).
Another advantage of the XenCenter is that it provides all the necessary features for upgrading the server license to higher editions of the XenServer without having to upgrade the console. (Citrix, XenServer Documentation, 2008).
VMware ESXi 4 Server
VirtualCenter is the EXS server web-based management console which is also quite easy to use and navigate, although it has fewer features than XenCenter (Covisia, 2010). It is highly recommended to use if automation and high availability of VMs is a must have, but a separate license must be purchased. It also requires the installation of SQL Server Express which is not supported on a Vista machine (Jones, 2009). This adds significantly to the complexity of the environment as the Virtual Center console becomes a critical and central management point of the virtual infrastructure (Clayton, 2009).
Microsoft Hyper-V Server
The Hyper-V management console must be downloaded and installed separately on a Windows Vista machine or on a Windows Server, as Windows XP is not supported. Being a new product in the market, the information available is very sketchy, and the required software updates are sometimes hard to find and to understand. The CTRL+ALT+Left arrow keys must be pressed to exit the management console screen, which is extremely tedious (Jones, 2009). The Hyper-V management console lacks from performance and analytical graphs that are provides by VMware and XenServer free editions. The full SCVMM must be purchased to obtain all the nice features with additional license costs (Vugt, 2010).
4. Conclusion
Choosing the right Hypervisor to consolidate a server environment depends on a number of factors. The information provided in this report was based on features, functions, accessibility and manageability of the three leading virtualization products. However, before choosing, one should first analyze the planned workloads, the hosting environment, the cost and what expertise is available.
Having to choose, Citrix XenServer is very easy to acquire. The free version comes with the best functions and management features. It is also upgradable to the full version without extra consoles. It makes more sense to adopt XenServer if the administrative staff has good knowledge of the Red Hat Linux.
VMware being in the virtualization market for a long time, offer a more experienced, reliable and successful platform with more VMs to host than anything else. The ESXi server is ideal if the IT department has good knowledge of Linux and the underlying hardware platform is supported.
Hyper-V is gradually gaining popularity in the virtualization market. Being part of Windows Server 2008, has given it a competitive advantage over VMware and Citrix, and isn't difficult to foresee it taking over at some point in the future. It is the ideal solution for windows domain infrastructures and for those with strong in-house Microsoft skills.
