Virtualisation solutions in comparison
// by Matthias Rensing
IT & Management Consulting
Nowadays, virtualisation is considered one of the key technologies in business IT. Server virtualisation and storage virtualisation allow for a better utilisation of the hardware, allowing for cost savings and a more flexible delivery of IT resources. Similarly statements apply to the virtualisation of networks, desktops, and applications. The topic of virtualisation has been a concern for the IT world for more than 45 years. In AIX and/or UNIX systems it was possible already as early as in 1972 to partition the hardware resources of a system for the parallel operation of multiple operation system (LPAR mode). But it wasn‘t until Microsoft pursued the topic of virtualisation with energy with publication of Hyper-V in the version of Microsoft Server 2008 that the topic received the deserved attention within the Microsoft community.
IT cost reduction is driving ahead virtualisation
Through influences such as the financial crisis and the shift in energy, the reduction of the IT costs has gained more and more importance. Manufacturers are striving to reduce the energy consumption and to optimally utilise the space available in the computing centres. Increasing the fail safety as well as reducing the administrative effort are nowadays additional good reasons for virtualisation. Below, the three most prominent providers for virtualisation, VMware ESXi, Microsoft Hyper-V and Citrix XenServer are inspected and compared.
What‘s really behind virtualisation, in detail?
The core component of a virtualisation solution is the Hypervisor. A differentiation is being made between two types. Type 1 runs directly on the hardware as separate operating system or in the kernel of the operating system. Microsoft Hyper-V, VMware ESXi and Citrix XenServer fall into this category. Referred to as type 2 are Hypervisors that are running as an application atop of an operation system and thereby come along with a slight performance disadvantage. Applications such as VirtualBox or VMware Workstation fall into this category.
Three kinds of virtualisation
The three most common virtualisation methods on x86 systems are the full virtualisation, the hardware virtualisation, and the „parallel" virtualisation, in short, paravirtualisation. The differentiation between the three virtualisation methods is quite easy when system‘s access to the hardware is considered.
In a full virtualisation, the Hypervisor is running directly on level 0. For each virtual guest system, a separate virtual machine with virtual hardware resources and virtual BIOS is provided. The guest system is executed on level 1 and is separated off on the lowest processor level. Each resource access must be passed on to level 0 via the Hypervisor. The guest system receives its own resources which it does not share with other guests on the host. This is a relevant security characteristic since the guest systems cannot access one another on the CPU level. Since the hardware in this case is provided by the host to the guests already virtualised, a lot of different operating systems can be operated virtually.
In paravirtualisation, the guest system is running directly on level 0 next to a very streamlined Hypervisor. In order for this type of virtualisation to function, the guest system must know that it is operated virtually since - in this constellation - the guest system must pass commands that it cannot execute immediately on to the Hypervisor. Most of the time, this type of virtualisation is faster than full virtualisation since it runs closer to the hardware. Unfortunately, there are operational restrictions in this variant in the number of different operating systems. XenSever, Hyper-V and VMware support this form of virtualisation.
The most compatible guest system is Linux since here Xen is already integrated as DomU (guest). But the Microsoft Hyper-V Integration Components have also been integrated in Linux since kernel 2.6.32. The VMware solution has been called Virtual Machine Interface (VMI) since Linux kernel 2.6.22. For all other guest systems, there are drivers from all three manufacturers for paravirtualised guests.
Since 2006, both Intel and AMD have been offering a hardware-supported virtualisation in their CPUs (Intel VT-x b and AMD-V). In this, the Hypervisor is executed in a new, privileged level even below level 0 and is the intermediary to the guest operating system which now can run unmodified on level 0, and the hardware level. (Level 1) As such, even guests that have not been optimised for a virtualisation can be operated. Unfortunately, this kind of paravirtualisation is very slow since each command must be intercepted by the Hypervisor. Here, full virtualisation is the quicker variant since an optimised software code works directly with the hardware here. By now, hardware virtualisation is not only supported by the three large manufacturers, rather, it is also being demanded.
Configuration and management
The three large manufacturers covered here each have a management software for the administration of the hosts and their virtual guests.
Citrix is offering XenCenter even in its free version of XenServer. The client-based XenCenter can be installed on any Windows client and allows for access to any host, and in the for-fee variant even an interface to Active Directory to control permissions. VMware delivers the vCenter from the Essentials Edition and up. This vCenter must be installed on a server operating system or as virtual appliance.
However, for clients, vSphere is offering the same functions as XenCenter. For basic administration tasks, Microsoft has integrated the Hyper-V Manager into the server manager starting with Server 2008. For the advanced administration of larger Hyper-V farms, the for-fee System Center Virtual Machine Manager, in short, SCVMM is advisable.
When you are operating professional virtualisation, you should not forego redundant storage with performance. Each of the three manufacturers provides the option to move virtual systems - while running - from one host to another. The systems are located on a hard disk and/or LUN (Logical Unit Number) in the storage area network or, in short, SAN. All hosts have concurrent access to this hard disk. The host master controls this access and receives the sole write privileges to the partition with the configuration data of the guest systems. In a best case scenario, the storage and the hosts of a farm are distributed over several server rooms so that in case of a failure of one part of the building, the virtual systems can be moved - without interruption - to another host in the still functioning part of the building.
Except for the snapshot technology, there are only minor differences amongst the providers with respect to the backup. Basically, the backup via snapshot on the storage level is the safest for a disaster recovery. However, for the recovery of a VM, it is rather laborious.
- Microsoft-typical manner, the virtual hard disks can be backed up via Explorer from the LUN from the SAN.
- For VMware, a for-fee tool must be purchased separately for a professional backup.
- XenServer has the option to export the virtual system via Cifs Shell onto a Microsoft file share and to also reimport it from there. This process can also be performed scheduled via script and is free of charge.
Load-balancing and Green IT
All three providers offer the option to distribute the load of individual hosts or to switch them off to save power. If, for instance, the full performance is not needed at night, virtual systems can be consolidated onto a few hosts and the other empty hosts can go into stand-by. In the morning, the virtual systems are then redistributed to the other hosts upon increased load in order to achieve the best performance. This saves energy and air conditioning costs.
All three manufacturers support virtually all common guest operating systems. For older guest operating systems, VMware – as pioneer of full virtualisation – is somewhat ahead of the pack.
In terms of performance and other options, there are only marginal differences between Microsoft, Citrix and VMware. The deciding factor is the area of use. If virtual systems are needed for mapping a terminal server farm, then XenServer by Citrix is the first choice. But if a high compatibility for all operating systems is required, then VMware is the best solution. And if a large Microsoft infrastructure needs to be virtualised, then Hyper-V is the measure of all things. The technology may differ, but the functions are similar.